We stayed here in Garda. The location was pretty good. Not next to the lake, but only five minutes from it. It had an excellent pool area, table tennis and table football. It is a family run place and the staff were delightful. The owner is very much involved in the hotel and does all the entertainment. He is a real character, pretty talented and very personable. The food was mostly nice. One or two of the meals could have been nicer, but we felt so well looked after that I would go back.
Garda is about a third of the way up the lake so it means that you can visit just about any town by ferry from there. There are quite a few shops and lots of cafes right next to the lake. There is also a flat paved pathway that runs along the lake as far as Bardolino that you can either walk or cycle along.
Sirmione is right at the southern end and on a long peninsula. It has a very photogenic castle at the entrance to the town worth seeing - you must climb the tower to get the most amazing views. At the end of the peninsula there is the Grotte di Catullo which is a huge site with Roman remains. It is really worth seeing.
Again there are lots of shops to look around. We stayed here. Our room was in the same building as the reception and was lovely and spacious. The food was excellent and the bar and outdoor dining area look out over the lake so you can watch the sun go down in the evening. The staff weren't very friendly though and the pool area wasn't as nice. That said I would go back there as well.
If you are going in September I would seriously consider Sirmione as there is a thermal spring in the lake and some of the hotel pools are fed with this water, as was the hotel pool where we stayed.
Both towns are different, but I liked both equally.