My bra requirements:
1) must have underwire
2) beige OR black (I own more beige than black) - no fancy colors and no white, because they show under shirts. My teens, on the other hand, prefer colorful bras because they wear tank tops a lot and it's the "trend" to have your colorful bra strap show - but it can't be a nude strap, because that's tacky! Hee hee. So they load up on pink and purple and turquoise and other fun colors.
3) no lace (although I do have one with lace; the lace must not itch)
4) I'm not sure if it's a molded or contour cup - not sure I understand the difference. But similar to the one you tried on at Nordie's. Lots of times I see these advertised as "tee shirt" bras because they aren't supposed to have any obvious seams showing through, and eliminate nipple problems. I don't find that they add much extra bulk. You definetly DO want to try them on under a close-fitting tee so you can be sure that you like the shape they give you. I don't want pointy cone-shaped breasts, but i do want some shaping.
5) I prefer "full coverage" bras for every day. The Nordie's one does not seem to be full coverage. Full coverage bra will reduce spill over/"heaving busom" effect. I do have one or two bras like this though, for when I want heaving busom effect (romantic dates, for example - but NOT for work!)
Your hot weather question: cotton is definetly going to be cooler. But I find it just doesn't give enough support and rarely holds up for very long.
Try Chantelle - they have a bra that is designed to keep you cool in hot weather. I have this bra, and it lives up to its claims, although, as I've mentioned before, the bra has developed sweat stains on the outside of the cup. What this bra does is wick moisture away from your skin and out to the outside of the bra. (No, your top doesn't end up wet! hmm, I guess it wicks the moisture to the inside of the cup?). Anyway, after wearing this bra in very hot summer weather, it developed stains that won't come out. It's all cosmetic though and I still wear it. It's very comfy. I can't stand sweating in bras, so it's worth the trade-off.