Handling is kinda bad after getting new rear tires - change the fronts or the brand

[new2bxstr]

I changed my PS2 rear 18s for Sumitomo HTRZ IIIs but left the PS2s on the front (have about 50% wear).

Well, my once solid as a rock, super-responsive Boxster is an oversteer mess (or maybe it's understeer - it's just kinda all over the place).

Do you think I should replace the fronts with the HTRZ IIIs or just replace the whole set with Michelins, like the PSS tires?

I can't figure out if the bad steering is from the HTR ZIIIs or the the fact that I have two different brands, with one more worn out than the other. The steering was fine before I replaced the rears.