Waaay back in the olden days when bass cabs were really just guitar cabs with guitar drivers, a bigger driver moved more air & 1 15" driver with a 4x10 was a damn sight lighter than 2 4x10s (3 less ceramic magnets to carry).
These days it doesn't really matter if you're using 5" or 15", you can get the same frequencies covered (Phil Jones cabs are all 5" drivers).
Mixing drivers is just like getting a Hartke cab & "pairing" it with an Ashdown cab. They both sound different to one another. You don't know how it'll sound until you use the 2 together. Using one without the other will sound different again.
If you like the uncertainty, then mix drivers/cabs, it might sound good, it might not. If you find a cab you like, then you just add another if you need more volume.
What size gigs are you doing? Do you need a bigger rig than a single 1x15, 2x10 or 2x12?
For how much you're looking to spend, you could get a good 2x12 & whichever head you fancy & find that you have a rig that will cover almost any venue.
I don't know why Hartke don't do a 2x12 cab, though they have 1x12 cabs. I'd happily buy Hartke just going on the great customer service that they've shown on this very forum.