Yep, agree with Tilly, been to both.
Garda probably has more/better ways of getting around the lake; Como you're more reliant on the ferries to move around.
T'aint cheap neither - in both.
Or Maggiore, all good. The excursions available may vary - from Maggiore you can go to eg Zermatt, from further east they’re more likely to offer Venice.
Sorry for the late reply re Sirmione. Yes, it is lovely or would be if there were no tourists. When we went it was absolutely heaving with them. We walked around for a bit until we could stand it no longer and got the first boat out!