To be honest your choice between the two would come down to your personal preferences. I would seriously recommend you'd go with what look you feel most comfortable with, and then have a look at which ones (within your budget limits) that has the highest quality externals. This is because it's always easy to swap the internals if they're bad or need replacing. The externals is always what you're stuck with, and upgrading them can prove to be costly if even possible. Many manufacturers aren't that compatible when it comes to externals, and such replacement parts aren't always that common.
For internal upgrades, you shouldn't worry too much. Sure there are more Armalite-type parts available, but replacing everything inside the SIG should prove to be quite easy aswell. The KA 556 even uses an Armalite-type hop-up unit. The fire selector-assembly isn't as bad as people would want it to be. If you know your way around with a screwdriver, then you can't get it wrong. It's really self-explanatory, tho I will admit it can be annoying the first time or so. What you also get with the SIG is none of the v2-related design flaws. Takedown-wise, there are always differences from brand to brand, although many things are similar for the most. The price you pay for slightly more annoying takedown at the fire selector, is that you get a ambidextrous fire selector, and in my opinion a better one that what you find on the v2 gearboxes.
If you can get your hands on a VFC HK416 then I can tell you it's SWEET!!! This is especially the case with the externals, but the internals will also hold for a while. I'm not really impressed with what VFC choose to put inside their guns, but that can easily be replaced when needed.
For budget-416's I'd recommend the Jing Gong FB6621. It's just amazing what JG managed to squeeze into a gun at that prize! It even comes with a mosfet (non-braking) and is of course full metal.
If you want a 556, then the King arms one comes with electrical blow back! (can also be disabled if desired) It has high quality internals and externals. You can get it with the civilian-style handguard, or with the front RIS handguard. It's also available in a shorty version that I would personally go for if I didnt love my 552 that much. The shorty only comes with a RIS handguard as far as I know. Sadly the good looking stock on these guns have some vobble. Of course it allows for fast stock adjustment, but mostly it'll probably annoy you. Magwell is also slightly tight for certain STANAG mags.
For more affordable versions there's the Jinpeng, which I personally know very little about. All I know is that it's full metal, and can probably be expected to be on par with most current ACM-clones. Only specualtions about quality tho!
The Cyma version I've yet to get my hands on, but newer Cyma guns have proven to be quite good! Do some research if you consider any ACM-clones anyways.
"Everything will be OK in the end. If it's not OK, it's not the end."