The better one is what works best for your kitty to be honest. Clavamox is a great antibiotic for upper respiratory infections, but can cause upset stomach or diarrhea (my cat Jason gets both - he starts out ok on the Clavamox, but towards the end of a course his appetite drops; also if I give it to him on an empty stomach, he pukes). Doxycycline is another great antibiotic for URIs (it's the most commonly used antibiotic for URIs at the shelter I work at - Clavamox is typically used with more severe URIs, or if it didn't respond to the Doxy), but can also cause vomiting (neither of my cats can tolerate Doxy). Orbax is another great one, although it's typically prescribed in conjunction with Doxy or Clavamox (at least at the shelter I work at, not sure about other vets) - it seems to be easier on the GI system. I regularly treat Jason's chronic herpes/URI flares with Clindamycin, an antibiotic that's really meant more for dental infections and the like, but Jason tolerates it really well and it does the job.
I hope Fritz feels better soon!