20/25 c is quit a range to cover. In 20c water, doing training for hours, when below it's colder (16, 12, 8c) then a 5mm open cell (medium density) inside, nylon outside is nice for my skinny tall body.
When I'm doing competition I obviously want less suit, and since I skip in water preparation, I have only 3mm. Now if the water is constant 20 all the way down, I would choose to have 3mm + vest. Now if you want comfort, more then super low drag performance, I suggest coated cell inside (easier then open cell), super stretch nylon outside, medium density neoprene. Ask for a low forehead cap (to keep the sinuses warm), and maybe an extra 3mm vest (same material).
The nylon outside makes the suit endlessly more durable, in trade for some flexibility. Something that good lubrication medicates. Smooth in and out is indeed more comfortable, faster and a bit warmer on the surface, but it will rip open very easy and spoil your day.
Also you can lend it to a friend knowing he will not break it instantly ..
Oh and there is also the sandwich option, smooth in, smooth out with nylon in between.
I have such a suit, from 2000... and it's very durable, but it is less flexible then single lining 5mm (mine was 4,5mm - 1,5mm more then the 3mm). So it is size critical. I think I may ask for a sample of the latest fabrics since I think the sandwich may have been improved with Higher stretch nylon than my OLD Beauchat Apnea 2000 suit had. Actually I think it's a really nice option in colder and windy climates, where you want smoothskin, warmth and durability.
Yeah choosing is difficult. If it's your first suit and only one, I would go for something durable, warm and comfortable. Good stuff for endless hours of recreational diving, learning the ropes without the fear of damaging your investment.
In Sweden in 19c water I was happy with my old standard 5mm (open Cell in, nylon out) for 1,5 hours without shivering. A 3mm would have me shiver within 5 to 10 minutes, if I skip food in preparation of a competition dive.
Being a bit on the warmer side helps to save energy.
In Egypt I dove in 26,5c water with just the 3mm upper piece (Nylon in, smooth out). The full 3mm suit was too warm.
It's funny that just 1c difference is easily noticeable in water compared to 1c change in air temperature.
I hope this is helpful.
Kars