If you're of age, you can buy from them without a sweat, I'm pretty sure they do check for ID.
Both are alright as starters, but I would probably go for the JG just for the sake of modularity, I'm not a fan of M4's by all means, but they're the most common AEG out there, and that being a Marui clone makes it all that much easier to convert it into a full metal AEG and upgrade it in the future. ICS tends to be a bit unforgiving in that sense.
Though, Keep in mind, price does reflect quality and durability in some cases. If you can, you could always save up for something along the lines of VFC or G&P.