Fi3re is right, it's more difficult to find aftermarket parts for any gun than the M4. M4's are so common you'll find all sorts of stuff for them from every airsoft retailer. After market parts are available for the sig 551, RIS handguard, top rail collapsable stock ect. but they will be harder to source than for any M4. I personally like the look of the SIG and don't want a lot of crap on my gun that I don't need or use for the sake of it. But thats just me I'm a minimalist and I want to keep my gun light. That being said the Sig comes with removable rails for the handguard if you want to mount a flashlight, PEQ box or vert grip ect.
I won't try to steer you in any direction just say that I am happy with my gun. I think both guns are good and you'd be happy with either.
Edit: if you can try to get your hands on both of them, pick them up shoulder them see which one feels most comfortable to you.