I always used flatpicks, mostly Jazz III/JD JazzTones size. I started using a thumbpick a couple of years ago after watching a Thom Bresh DVD about Travis picking. If you play Chet stuff, Travis picking or fingerpicking in general you can't at least try a thumbpick. The advantages are pretty obvious, you just need to find the right thumbpick for your needs and practice. My first thumbpick was a Dunlop 9022R (plastic tortoise) I bought in a local shop. After a couple of weeks I was able to use it comfortably for Chet stuff and fingerpicking so I used it for a while. I tried other thumbpicks during the time (Herco, Ernie Ball etc.) but I eventually came back to that heavy thick Dunlop. My musical journey lead me towards country chicken picking/hybrid picking so my right hand got into a transition phase. I realized pretty soon that chicken picking was definitely doable with a thumbpick, but some of those fast alternate picking lines were hard to play so I found myself going back and forth from a thumbpick to a flatpick depending on the music I wanted to play. It worked of course, but I HATED it. After reading great things about Fred Kelly's Slick Picks I tried one of them and everyhting changed. After a couple of months of practice I was able to use it for everything, from Chet stuff to chicken picking, alternate picking, even strumming. I didn't look for something else until I accidentally found a Fred Kelly standard pick at home. I tried it and, in my opinion, it's even better. I'm sure there are other good thumbpicks around but I found the one that works for me. Recently I started to play some Brent Mason songs and I've seen how he replaces every upstroke with the use of one of his fingers (mostly middle finger) during single lines. I'm mastering this technique and basically it makes you able to play with any thumbpick you want. This is just my experience, I hope it will be useful for you.