I have Icon padded shorts to wear under whatever pants I ride in, which is nice since they'd likely stay planted underneath no matter if the suit/pants moved around some in a crash.
A 2 piece suit seems like it may be more versatile since you can wear the jacket with other types of pant, and vice versa. Pricewise, I'm not sure how they compare to 1 piece suits as I've never sought out a 2 piece - but seems for an occasional trackday guy, the 2 piece would give you more options for your money. The hypersport pants look plenty good enough for some novice tracktime, and I wouldn't worry about kneedragging too much your first day....that will come in time as you get better with body position and corner speed.