First, the piston system does one thing significantly better than the DI system - it makes your wallet lighter.
The Sig at wal-mart is a DI gun with no sights, just a picitanny rail and low pro gasblock. It's a heavy barrel (looks chrome lined), non-free float with a fixed stock and 10rd mag. It would make a great varment or predator rifle. I do not recommend this rifle as a first AR b/c most people end up wanting a lighter weight carbine.
The "Remington" is probably a Bushmaster M4gery. I do recommend it as a first AR: chrome bore for easy cleaning, M4 profile barrel saves weight (I'd prefer a light weight barrel, but whatever), has a fix FSB and removable carry handle (basically you can shoot it as is and add optics later). Bushmaster isn't the flavor of the month so you probably will have 100 other recommendations on other ARs not available to you right now and/or priced well above $1000; then probably 3-5 people will out right bash them with no first hand experience; then maybe 1-2 people with a legit problem that had to be fixed under warranty.
If it is a Remington R15 (camo) I'd say pass b/c the barrels and bolt carrier are not chrome lined (at least they used to not be). I mean the barrel is ok not lined; it's not much harder to clean, most people will never wear out a non-lined barrel, reliabilty will be there if kept moderately clean; but the non-lined bolt carrier group is a no-go for me b/c I can see that being a big reliablity issue after a high round count and the carbon is hard enough to get out of there in a chromed carrier.
ETA: I recommend buying vs building for a newbie. Buy it, strip it, clean it and use it. After you see how everything works, then you can build what you want from there on. You really don't save much anymore building vs buying on a basic AR anyway now a days. My 10.5 SBR will be in the mid $700 range before tax stamp, and that was using spare parts and looking for deals; my 20" A4 ran the same doing the same. I won't even get into this retro faze I'm in b/c both A1 will run ~$1000 a piece built. Also you may not be familar with the transfer process or FFLs that will transfer in your area yet to purchase lowers by themselves.