Which Growlithe should I evolve?

Transfer the 48% growlithe.

Evolve the 91% growlithe, you have enough startdust to power this one up.

If you have a lot of Growlithes in your area, you'll be swimming in higher level Growlithes with bad IVs as you level up to 30...I've caught over 350 so far. About to evolve a few 29s/28s/27s...at my next evolve party.

I'd guess 91% isn't that high for a lot of people neither to spend dust on since Arcanine isn't a good defender (defenders need more CP) and Arcanine's CP is now too low to stay in high level gyms (2600CP min).

That said, you can evolve both, but don't power up the 48% one (or even the 91% one) since as an attacker, anything works fine really.

If you still want to get a top fire attacker, I'd probably wait for a 95%+ one (43/45 stats...) before powering up. You still have to get the right move set (no bulldoze) so you can decide on powering up after evolving it.

I'm considering powering up a 95% (Lvl 19) Growlithe if it evolves and gets Bite/Fire Blast (near 750k dust on me currently).

If not, I figure more stardust for other mons if anyone fits my criteria (don't feel a rush as I already have many usable Arcanine's with FT and FB). IVs make very very little difference in actual combat.

Personally, I would evolve the 91% IV one. If you get a crappy moveset (i.e., Bulldoze), then evolve the other one.