Welcome to the forums, mate!
Of those two, difficult to say... those shotties have a pretty good rep for cheap guns, but you'd best get someone who has actually had one, and for a while too's, opinion. I can't say I've ever heard anyone raving on about SRC M4's, they're better known for their G36's, but it also depends on what generation of SRC's manufacture it is: Gen 1 are fucking useless, Gen 2 need upgrades to be skirmishable for a good long while, Gen 3 are very good.
If you're thinking that, after a while plinking in your garden, you might want to check out a skirmish, then the M4 would probably be better than the shottie, but then again, you may be better renting a gun than taking something crap for your first taste of the sport, so you don't get put off.
Just a bit of general advice: any website with bbguns in the url should be avoided, such as justbbguns, onlybbguns, etc.. They're much of a muchness and known for selling dud guns, terrible customer service, and making exaggerated claims and/or telling downright lies about their guns for sale. Have a look in the "Places To Buy" section here and you'll find plenty of reputable retailers.
