okay, now for reals. for years i've ridden in a full, one-piece leather (custom made) suit. it's amazing, comfortable, armoured, and unbeatable if i ever go down (touch wood). yes, it's brutally hot in there when it's hot outside, but you pay to play, eh? it's also super super comfy when riding in reasonable temps, great at speed, for long days in the saddle, etc. etc. there's good reason people wear leather on motorcycles - it works. i love my leathers.
1. 1 piece. the two piece ones are stupid-looking, and the jacket is too short to really wear separately, and you'll *never* wear the pants separately (cut wrong and stupid-looking).
2. Look, you're buying leathers. If you want comfy 'n cool on the beach, buy a thong. For leathers buy protection not ventillated crap.
3. Test 'em out. Or buy custom, it'll last for the rest of your life if you don't abuse it. And they can be repaired if you crash.
4. Buy the right size - talk to the salesman. They do get a little looser, but not much. Don't "buy loose" - it needs to fit to protect you, man. If it's floppy and baggy it won't spread the impact load well, and will bind and do bad things if you crash. When you hit the ground, you want the leather to 1) spread out the load to avoid internal damage, and 2) slide smoothly on the asphalt to prevent tearing. For both of these, you want it to fit smoothly to your body.
5. New. It's an investment in a healthy dermis, undamaged body and comfy riding. do the right thing.
Good luck Leatherman.