Has a few other have already pointed out, both are close to be same style Pro-Street. The Mastiff is the soft tail and having an 05 Mastiff, I have enjooyed the ride with not problems over many many miles inlcuding a 700+ on day jaunt. Never a problem.
Do check the Rocker Boxes, if the TP are there, some have had issue but many other have not. I still ahve the TP Rocker Box on mine (thought the valeve train were changed over to S&S but the box are still TP.
Not sure but I think even Raywood amy still have the TP boxes and he is close at 90,000 miles on his 05.
As my good friend TooManyBikes points out, try to get history, ask for the VIN and you may still be able to get some history on it. The VR on some were changed, the strut bolts and maybe a couple of other, many of these were on the 05 models, but some crossed over into other years.
Either way both are great bikes and the prices seem about right for mileage and years, barring that they are in good shape.
Good luck and keep us up to date on what you end up with.