My son has an M4 so I can do a comparison...
These are both set up in a very similar configuration: 7rd tubes, 6rd side saddles, RMR mounts, WML's.
My 1301 as shown, 7 +1 loaded = 9lbs 2oz.
The M4 as shown, 7 +1 loaded = 10lbs 3oz.
AOL is as follows:
1301T is 37.25 inches w/ 12.125" LOP
M4 fully collapsed is 35 inches w/ 9.25" LOP
Mid-position is 38 inches w/ 12.25" LOP
Fully extended is 40 inches w/ 14.25" LOP
Camera pics tend to distort perspective but here's the M4 stock positions against the 1301T for what it's worth.
In my opinion the mid-position on the M4 stock makes any size difference a wash. Where the issues arise is with the weight and ergonomic differences... The Magpul SGA stock feels much better to me than the slightly awkward pistol grip of the M4 and the extra pound of weight is very noticeable. Plus building the 1301T will most certainly cost you less than blinging-out the M4.