+1 for rounds
Rounds are just more versatile. Casual or dressed up, you can never go wrong with rounds. As far as overtones go, it's best to get out and try some on because not all tones look good on everybody. Plus getting out there and trying them on gives you a chance to see what's out there and that way you can narrow down your choices. There are quite a few online retailers to actually purchase amazing quality pearls from and know exactly what your getting for your money. Unless you have a trusted retailer already, I've just seen too many people buy shell pearls thinking they are getting the real thing.
Also when you are out there looking, try on different sizes to see what you like. I always loved Wilma's pearls on the flintstones as a kid but I didn't like the look on me. Which was sad because I see large pearls on other people and I'm so jealous.
One other piece of advice is to get some really nice quality freshwater pearls as they are all nacre, they really last a long long time. Akoyas are nice but they have a bead nucleus, so over time the outside nacre can wear away exposing the mother of pearl bead inside. I have several vintage strands of akoya pearls that I love but my freshies get worn the most.
I hope you didn't mind that my post is a bit long winded, I just really like to see people get there money's worth and as you can tell I have a soft spot for pearls.