I prefer 20 round mags. I haven't used any of my 30's in years. Thirties are too long for shooting from a bench or prone and for me they just aren't practical. My favorites are the Okay surefeed 20's which are just renamed from the NHMTG mags and the exact same mag as the Colt 20's (Colt doesn't make anything anymore, everything is outsourced including the new Python) they just have a floorplate with different stampings. That Colt stamped floorplate costs double or more than the same exact mag with a different stamping. I guess some people just like to pay more for that pretty pony.
Lancer AWM mags are one of my favorites but they cost a little more than others. Pmags are good mags and lots of people will tell you they are the best. I like them but we don't know how well they are going to hold up after thirty years. Aluminum G.I. mags have been proven to last for decades. D&H mags are good to go as are C-Products mags. D&H twenties are curved and have a smoother finish whereas the Colt/Okay mags are straight. I have a bunch of Elanders, they are solid (made of steel not aluminum) and heavy. There were problems with some of the first gen Elanders in certain lowers either not locking in or not falling freely when released. The newer versions are really good though. Thermolds can be found for cheap because they got a bad rap and many folks call them "Thermelts". The term actually came from the Canadian Military contracting to make their own Thermold mags but they used a different polymer than the originals. Under heavy firing the feedlips would melt. That does not happen with U.S. thermolds. I've got dozens of those as well and never had an issue. They can still be found for six bucks each. They used to be $3 and they aren't a bad mag for range duty.
There are several new mags in the market now, Hexmag, A2 , ASC and others. I haven't tried any of those personally. Run from anything that says Promag, they are junk.
Whatever thirty round mags you end up with make sure they have anti-tilt followers in them and if they don't then just replace them with pmag followers and one more thing, you can never have too many mags. Just ask anyone who lived through the Clinton AWB.