I like a one piece. When it’s warm I wear just a 1 piece 3mm, as the temp drops I can add a 3mm or even 5mm shorty over that to keep my core warm without bulk and restriction on my arms and legs. When it’s even colder, I like a 5mm or 7mm one piece, with or without a hooded shorty or hooded vest under or over the full suit. This can give you a full range of water temperatures from the 80s all the way down to the low 50s with just 2 different full 1 piece suits and one shorty. I like this better than a FJ with jacket, because I can wear any piece alone or in combination with any other piece. With the FJ, if you only put on one piece you have either bare arms or bare legs.
For medium temperature waters, I’d suggest starting with a 5mm 1-piece and build out from there based on where you go and how you feel.