[Billfred]

Advantages I noticed:

-Toughbox mounting aside, there are only two types of pieces to the frame (C-channels and corner connects). I don't have to worry about freshmen getting more rail when we needed bumper--they're all the same now.
-It's free and in your possession right now.
-Drop-center 6WD is still doable...well, if you want to do that this season.

Cons I've noticed:
-Bolts only. (1618 riveted their frame last year to much success; I let out an obligatory small grumble when we couldn't do it out of the box this year. That said, you could make gussets to do the job if you wanted.)
-If you're going for something more compact than 28"x38" (or vice versa), you could be drilling new axle mounts.
-If you liked the low-rider approach in the past, tough; it's not an option this year. (After all, all...my...friends...know the low-rider.)

Even with the Kitbot legal this season, 1618 will all but certainly be using the C-Base. I can't find enough cons to the C-Base to justify spending an additional $189.95 (plus shipping, plus time) on the former. Your mileage, of course, may vary.