When i get my next set, it'll undoubtedly be the 4200s. They are lighter than the 4500s by 4g and hold about the same mAh.
Although I'm getting really good performance from the 4500s with WOT and at the end of the straights, I think the 4200s (with less internal resistance) will keep batt temps a bit lower with my prop choice.
Go with flat pack, since it will be easier to break the packs down in the future if needed. Use the $.75 option at the bottom of the order page. It's up to you on batt config (flat or standard).