Since you use the word, "invest"...
I suggest a USGI M1 Carbine. Why? They aren't making any more of them. The value will not go down. More likely, it'll appreciate. Ease of acquisition is unlikely to go up.
In contrast, unless there's another AWB, the only thing preventing you from getting an AR-15 is lack of funding. They are probably the most popular single type of rifle in the US now, and that doesn't show any signs of changing. Unless you get some special, rare variant, or there's an AWB with a grandfather clause that permits transfer, it will depreciate.
From a practical user standpoint, ammo costs are comparable now that cheap .223 has dried up. Either will work just fine as a defensive gun (assuming good functional condition and decent ammo). The AR-15 has an edge in terminal ballistics past 100 yards, but unless you're hunting, target shooting past that distance, or using it in a war zone, that's not very important, IMO.
Here's a pic of my 1944 Underwood to further whet your appetite:
And my gussied up 1943 Rock Ola:
Oh, what the heck. Here's my Colt AR-15A3. I have a different scope on it now, though.