All wetsuits make you faster. You are more buoyant and therefore create less drag. You will ALWAYS be faster with a wetsuit than without one. As for full sleeve or sleeveless, I say go with warmth first. Get the full suit so you can get in the ocean/lake earlier in the season and stay in it longer in the year. The sleeveless gives broader range of motion which can be a consideration. The difference in "speed" is minimal.
With that in mind, it is my contention that the *primary* consideration when purchasing a wetsuit should be fit. A wetsuit that doesn't fit will make you miserable in training and racing. Different wetsuits fit slightly differently: a medium ROKA might fit slightly different than a medium Blue Seventy simply because of the cut of their patterns.
Things to look for:
Too loose - The suit should not be too loose or it will hold too much water and actually slow you down. Can also bunch up and create chafing.
Too tight - Can impede the natural swimming motion, make breathing uncomfortable, and rub severely.
Trouble spots - places where, for whatever reason, the suit feels like it is rubbing when you move. Common trouble spots include the side and back of the neck and around the shoulder cuff including along the lats. In a sleeveless, the shoulder area can be particularly problematic due to seams at the arm holes.
Try on a number of different brands and find out which one you like. The LA Tri Club has ROKA wetsuits for you to "try before you buy" in a whole range of sizes for both genders. Contact Nir Tal at Bike Improve in Westwood to try one out.