Welcome to the forums!
I've played a lot of both Space Marine / Titan Legions (which Net-Epic evolved from) and EpicA (Net-EA is the same game and rules, just with some community rebalancing of the lists and a wider range of lists). To cover some of the differences that you're asking for:
Space Marine has extensive over-complicated individual special rules its units, with whole A4 pages of mixed text and background at times, whereas EpicA has a much more streamlined system of mostly common special rules. EpicA armies are more inline with current 40k.
Space Marine has a diverse and very extenive range of cards, counters and templates. EpicA doesn't use any, just the same small and large blast templates as 40k and blast markers (some people use dice to track these).
The Space Marine game system is ok and I enjoyed playing dozens of games back in my youth, but EpicA i sa WAY better system, with much more maneuver and tactics. There are blast markers which have a signifigant game effects.
Space Marine was developed by GW, EpicA had a lot of community involvement with rules and list development during it's creation, producing a better game for it.
Where I've seen people talk about playing on here and in real life I would guesstimate 75-90% of epic players play EpicA.
There are 10 or so EpicA tournaments in the UK a year, see whereas I've never heard of a Net-Epic or Space Marine tournament.
CaptainSenioris is in Edinburgh and plays EpicA, with a very large range of epic armies, check out his
log, it's inspirational.
If you don't have local players a good option is to get together two armies (painted if possible) and talk some 40k players into giving it a try. Getting to use titans and wings of aircraft in regular games can be a draw.
however as a slight consolation prize I did find my Space Marine stuff. So after some internet research I got me a set of EA rules and also the NetEpic ones.
