The thing with the mk2 vs the Mk3 is what your average temps diving will be? If your average is 50 and below, get the Mk3....If higher, get the 2....The reason for this is with the 3 if you get it and for 4 months out of the year its perfect, well the rest of the year its too hot and you must buy another undergarment right away or not dive. With the MK2, if its too cold, you can layer up underneath it and get buy without buying a thicker one(forever or at least for a while). If its too hot, you can take the sleeves off it...It just has a bit more flexibility.
Also, it really depends on you. Are you typically a cold diver? Colder than most you dive with, on par with everyone else, or warmer than most? How much do you weigh? Im tall and skinny and dove the MK3 today with Fusion, granted I was in the water from about 8:30am-night dives(except surface intervals and inbetween classes) temp was 47 at surface and 44 was coldest I read on guage, and I was a little chilled by the end. I didnt have anything under it though except a shirt and some light weight pants. Overall, I was extremely comfortable the whole day, but again, I get cold easy.....How is your body while diving?