Hello, I'm Dr. Deb.
I recently came online and see that your question hasn't been answered. I'm so sorry that you've had to wait for a response, but if you still need assistance, I'd like to help if I can.
When I've diagnosed a urinary tract infection in my canine patient, I'll typically give either one of the antibiotics that you mention.
In the absence of a culture and sensitivity, either of these medications should work well although there has been some resistance to Cephalexin these days. But, when I do dispense it, I dose at 375 mg twice a day.
So, Clavamox would probably be the better choice. The dose that I'd dispense for a 25 lb dog would be one 125 mg tablet and one 62.5 mg tablet twice a day (187.5 mg at one time in other words). A 125 mg tablet could be broken in half; however care would need to be taken since once it's out of the blister pack, the tablet will be affected by light and air.
I typically treat uti's for a minimum of 14 days.
Most dogs tolerate this drug well but I always advise that it be given with food to avoid stomach upset.
I hope this helps although, again, my apologies for the delayed reply. Deb