there are no advantages to running a stanag mag in a g36 aside from reusing m4 mags you already have or saving money on pouches because you don't want to pay more for g36 pouches.
A g36 hopup and barrel group are annoying as hell to deal with, just ask john from airsoftstore... and me cause I've dealt with hopup issues all year long trying to get it to work.
if you want to run m4 mags, get an m4. air seal issues are very easy to fix on m4s.
Beyond that, the choice is up to you, if you like the goofy bastardized look of the mtc lineup, then by all means, it's what you like.
The prices you have listed are not bad, I know what vendor you are looking at and pricing is pretty good from them for a few lines. If I were to choose to buy an echo1 though, it would be more along the lines of the SOF lineup.
If you want a g36 because you want a g36, then look at the JG one.. E1 MTCs are just JG guns with the converted bits.
If you want an E1 AK, look at the CYMAs... that's who makes them.
E1 just rebrands guns and gets the licenses to brand some guns like the SOF series. Do a bit more digging and you could get the exact same gun that E1 sells for less if you find out the OEM for that model, the only difference being a few things like engraved trades and more markup.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.