SO I am having a hard time deciding which mag to purchase and hope someone can give some insights...a lot of 30rdrs, 40rdrs- affect prone shooting?, or a betaC mag - is it true these can be left fully loaded indefinitely??--

Thanks for all input

USGI 30's..........AND BETA-C's, LOTS OF 'EM!

Edited to add: Most 40's are crap. Check the mag-faq. USGI 20's are probably the best for prone shooting, and it doesn't seem the height of the beta-c would present a problem either.

Personally, I'd get ten or fifteen 20 rounders and as many 30 rounders as I could find room for - they're like potato chips, you know.

lots of 30s

couple of beta-cs

30's

USGI 30-rounders should be the backbone of your mag collection.  And brand new, USGI-contractor 30-rd. mags just dropped in price from 30-50% for non-State-AWB people.  

Get a few jungle clips ($5-8) or other mag connectors to bind 2 30-rounders together.  

Also get a few 20-rounders so you get a feel for them.  I like both the USGI 20s and the Thermold-Master Molder 20s.

Beta Cs are much more attractive now at $250 than $600, or even Ruben Mendiola's longtime price of $389, before he sold out his supply.  Never tried one myself.  Maybe you could hook up with an ARFCOM member in NV that has a Beta-C, to try it out.

You'd better offer to fill the mag, once for you and once for him.