With the uniform quality available these days in the brands mentioned I think it comes down to the little extras included in the deal like Crayford focuser, collimating eyepiece, etc.
My advice is not to go too big too soon an 8" will blow your little eyeballs out of their sockets after the 70mm, a 10" would be dangerous
If you stayed in this range then you would have heaps for some of the little extras that just make life that bit easier.
Cheers