Clare, I live in San Miguel de Allende (SMA) and have lived here for 5 months. It is a fairly nice place but a bit more expensive than I have heard of living in other Mexican locales. Just for an example, there is small place called Tequisquiapan (not far from Queretaro and about 3.5 hours north-east of Mexico City). I have been told that is quite easy to find a 2 bedroom furnished house for between $400 and $700 US per month. In comparison, SMA is typically over $1000 US per month. That is not to say that you cannot find places for less in SMA. That is certainly possible, but not as easy as I have been told it is in Tequis. This disparity in rents, I believe, is due to the larger numbers of expats living in SMA. Guanajuato apparently has a lower number of expats so that may result in lower rents. I have also heard that Guanajuato requires you to be in good shape due to the terrain. I was there, although only for one day, and did not find it that much different in terrain than SMA. In fact walking around the city center was better because they use flat bricks to pave the streets rather than the uneven cobblestones they use to pave the streets in SMA.
You are coming in August which is one of the hottest months. That is good as it will give you an idea of the worst it gets climatically. How you experience this will obviously be subjective. Two weeks is not much time to make any kind of decision. But it will get you started. I would suggest just moving here and renting and then, in time, visit some other places in Mexico and then making a decision as to what location best suites you.
For me, the dryness is a bit of a problem for my nose and for sleeping at night. I am told this dryness will last for several months. For this reason I may decide a more humid location might be the best.
SMA has many social venues for expats that you can take advantage of. Guanajuato may have these as well. I met quite a number of expats visiting Guanajuato the day I was there.
In general, I can say that life is pretty good in SMA. But, likely, I will not stay here. I prefer places that have lower numbers of expats. There are some 13,000 living in SMA. From what I can glean from other posts there are fewer expats living in Guanajuato than SMA.
I have a cat. He came with me from Canada and has lived in Cuenca, Ecuador for 2.5 years before coming here. I live in a house that has high walls around it and a large garden. He spends a lot of time outside especially at night. I sometimes let him out in the street by himself. There are other cats wandering the streets as well. But most of the time they are not around. The few dogs that are around don't seem to bother him. When you move if you decide to fly to Mexico you will be faced with some decisions to make. Having 3 cats, I would think driving would be better for the cats.