not really knowing your experience it may be a little hard for you to decide. i am a bit of a newcomer so take what you can.
i am 5'11" and flux between 200-225lb a few times a year. i used a mamba 8 and an 8.5 last summer. the 8.5 felt a little more stable but not quite as easy to throw around. after shopping and over thinking a great deal i ended up buying a pyrahna ammo-L.
in hindsight i think whieghtwise i could have gotten away with a medium but thought a large would be better suited for the ocasional rec paddles i do on calm water (sitting a little higher to help in shallow water, little more floatation for gear wheight, ect) both of them i find extremely easy to get in and out of. you are going to have to customize the padding no matter what so i think the best thing to do is just sit in them at a shop or rolling session. as for which one i suggest,,,, depends on your skill level. the ammo is a little better at the technical stuff but the mamba is a bit easier to roll and feels a little more stable on its side.
hope this helps