3.5mm vs 4.5?

i wear a 5/4/3 year round and would never go back to anything less

been using hand me down 4/3s for the past year. just scored an excel 5/4 with the hood and life in the water and immediately afterwords has been a lot more enjoyable.

5/4/3 most of the time here - 4/3 when it is "warm", but the questions to answer are do you run warm or cold? How long are your average sessions? Do you go out on the really cold dark days? Booties?

Most of the company's measurements vary...not sure how they are getting .5 mil differences - I'm guessing it is marketing crap. Compare a good suit, like Patagoina to a regular suit Oneill and see if the 4mm in each are the same. They weren't on my suits by both brands.

Just try it out and see, that is the only way to tell. If you are cold, now you have a suit for Santa Cruz. Keep buying thicker for here until you say shit, I wish I had bought a 5/4/3 five years ago.

I've got a 4/3 oneil, last years mid level model, front entry, really flexible. Havent been cold in it yet.

I use a quiksilver 4/3 and it is super warm. For dawn patrol in cold days, I got and xcel PolyPro Hooded top, which keeps me warm enough.... with this I feel like I've got a second wetsuit for another 60 bucks.

Skip the 3. If you want an all around suit go 4/3 and up. I have a 4/3/4 hooded that's nice and warm, even this morning. If you're too hot in a 4/3, ditch the boots.

I wear a 5/4/3 nowadays. A 4/3 on warmer days only. Much better!

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I have a 4/3 Excell X-Zip. Super warm & comfy. On cold days I wear a detached hood. Only drawback is that it has not been very durable. Serious signs of wear and degradation started showing up after about eight months.
My friends are telling me to spend the money and buy a Patagonia R3 next time. They say that they're warmer, lighter, thinner and dry faster than any suit they've ever had.

O'Neill 4/3 for me. It keeps me warm enough with booties when water temps are 50-55 (which is the typical winter water temp range around here) - I never wear a hood. With temps 55-60, I'll ditch the booties.... and with water over 60 I'll even ditch the wetsuit if the air temp is also warm enough - but that's pretty rare.

I was a longtime believer in just needing a 3/2, but now having a 4/3...I'd never go back. My RipCurl eBomb was the best suit I've had until I got my Buell suits (both 3/2 and 4/3). I could never do the hood thing though...too hot.

The Isurus 4/3's are pretty warm and super light, their i-Elite will get you through all year instead of having to buy a "warm" and a "cold" suit.

the human body adapts to cold surprisingly well.... wear less rubber and you will get used to it and be able to stay out longer as the winter rolls on. I like to minimize rubber to maximize flexibility and paddling power. Save the hood and gloves for the super cold windy days.

I have an o'neill 4/3. I have never been cold in it, even though it is a back zip. One of my buddies just got the new mutant 4/3 and says it is toasty as hell in there. I feel like wetsuits have gotten a lot warmer and more flexible, just in the past 3 years or so.

I use closed-cell a 3/2 year-round and am warmer than all y'all.
It also allows me to move like a neoprene cheetah. /does karate move.

(On the flip side, British wetsuits are like British cars. They look cool, but spend too much time in the shop.)

I just picked up the 3.5/2.5 Psychofreak at the O’neill shop on 41st about a month ago and I am super STOKED! Truthfully I was a little apprehensive about the thickness of the suit at first. I surf at OB regularly during the winter months, so I need my suit to be pretty warm. The research I did prior to the purchase led me to believe that with the new technology O’Neill has, this suit would hold its own. Well, I can attest after several days out in this suit that it is one of the warmest suits I’ve worn, and I am accustomed to wearing a 4/3. The Firewall interior goes half way down your legs and covers your chest and back and the suit is extremely stretchy in the arms which makes it very easy to paddle. On the colder days I'll add my long-sleeve 8oz.Thermo rashie and for the really cold days I'll wear my O'Neill Sport Cap/hood. I strongly considered a Mutant for it's versatility, but the hood option was too constricting for me.
hope this helps, but as @snowyplover mentions there are many different variables present in terms of comfort-ability level from one rider to the next.
By making the trip to SC I saved a ton of cash too!

I've found that suit Brand preference has a lot to do with body type. I'm quite tall and thin and Oneill is really the only brand I get a really good fit in (size MT). I also have very little body fat insulation. I have a psychofreak 4.5/3.5 and its super warm. To step it up I wear a vest/hood combo underneath if I need that extra warmth http://www.oneill.com/#/men/americacanada/collection/men/wetsuits/surf/c...
. This suit is rated 10-15*C on their website as is one of the options you listed above. I think if you go for the 5-10*C range it would be too much.

I wear the Patagonia R3 hooded and non hooded all year long. Great suits. The wool makes up for the 3mm and the suits last longer than other brands. The wool requires more care/rinsing, etc but is well worth it. I have been a product tester since day 1 of Patagonia wetsuts and they have come a long way since they began making suits. Their ironclad guarantee is also an added bonus, as they very much stand by their product. Wetsuits are nowhere near environmentally friendly, but Patagonia takes a lot of steps to make a very eco friendly suit, for what that is worth to you. I find the 4mm Patagonia R4 to be too warm for around here, except for the really big and cold days or when going up north. I also where 5mm booties, as I find my feet to be the first thing to get real cold, and once this happens, I am on my way to getting stiff and cold.

If I didn't wear these suits, I think XCEL makes great suits for the cold water, although I find their suits to last just about one season with very regular use.

Thanks - The Beard

Bought the 4/3 O'neil Mutant last December. I wear it with the attached hood always. It has kept me feeling warm for the first 12 months. I am now looking to buy the undergarment thing because i am starting to feel the elements a bit. I have worn the suit about 100 times. I intend to wear this for another year for a total of two years. I'd pay more for a my next suit if i knew it would last more than one good year and the second year just getting by........Not sure if the more expensive Patagonia or Isurus will keep me toasty for 2-3 years or are they a season and suffer like my O'neil......