subframe:
Pro: evens out the weight of the hoe, and the stress it makes to spread it out over the sub-frame mounting points.
Con.. may reduce ground clearance, cost.. etc.
3pt:
pro:.. easy on.. doesn't kill ground clearance.
con: probably leave you with a broke in half tractor if you actually do any meaningfull work with it.
3pt hoes generally can't be uses as vigorously as their subframe cousins due to the way tractors have their frame / castings setup. IE.. a 3pt hoe tends to get less work done if you don'y want to rip your rear end from your trans.. or your trans from your engine..
soundguy