Well, probably to no one's surprise, I'd go with the Colt.
Keep in mind that I have yet to examine either specimen, but some features to consider are:
A. M4 enhanced feed ramps. The Colt likely has them, the Bushmaster does not.
B. M4 double lined handguards. The Colt likely has them, the Bushmaster does not.
C. M4 side sling mount. The Colt likely has it, the Bushmaster does not.
I would also want to know if the Colt has a chrome lined chamber and barrel. If they are using contract overrun barrels, they would likely be chrome lined. This alone would not be a make-or-break issue, but it would be a minor factor and I would want to know before purchasing.
A post-ban Colt still retains it's value better than a post-ban Bushmaster. Just look through the Exchange Forum for evidence of that. Monkeying around with it and swapping parts may negatively affect resale value, however.
As has been said, a Colt or any AR15 that says "M4" on the side is certainly desireable and, depending on how many are eventually produced, future laws, etc, may even eventually attain some degree of collectability.
As far as the barrel twist rate, most shooters will never notice a difference between 1/7 and 1/9 twist rates. If you are the type of shooter who sits at a bench and expects to shoot teenie tiny groups on paper targets, then you will better off with a 1/8 or 1/9 barrel twist rate. For drilling pop cans, steel plates, etc. out to several hundred yards, either 1/7 or 1/9 will be suitable. 1/7 works great for me.
Lastly, a simple perusal of the forums would unquestionably indicate that Bushmaster rifles are plagued with considerably more quality control problems. Why they can't get a handle on some of their long running problems is beyond me. Colt certainly is not perfect, but you're far more likely to get from them a product who's fit, finish, functional performance, and reliability are second to none.
All of which adds up to the extra $75 for the Colt being money extremely well spent. Upgrading the Bushmaster with the side sling mount and true M4 handguards alone would cast more than that. The rest is just a very thick layer of icing on the cake.