We did go (so did the ailing Grandpa--just 2 days ago--after our return--Bless His Heart!)
We all met in Leon aiport (BJX) and private car took us to San Miguel. SMA is about 1 1/2 hr drive. Plenty of time to explore our quirky hotel rooms w/shared LR/BA. Casa del Misionero Hotel
They have a good spa if you get a special but the hotel doesn't make me want to return. Decent beds but very weird everything else. Poor bfast but great fresh juice--on the terrace! Good location but with a little work, you can do better.
Checked out lots of walking options around town. So many gringos and oodles of hills. We stayed 4 nts due to late arrival but 3 would do fine. We enjoyed the Mercado in SMA.
We took a bus (luxury, express bus, my hiney!! The Local stopped about 12X and took 2 hrs. Something was lost in translation w/tkt agent...)
Guanajuato is much more vibrant town; about twice SMA size. GTO has twice as many hills, steps and up streets. Cool funicular, good food and vibrant square where we had a house. Exquisite Home in Downtown Guanajuato... - HomeAway Guanajuato. Fantastic! stone entry up 1 floor, 20 ft ceiling level w/kitchen, BR, BA, LR and DR. Multiple set of French doors to plaza below (very noisy BTW) spiral stairs to 2nd BR/BA, lounge plus 2-level terrace in a jungle of plants! Old restored place oozing charm. She cut us a break when I told her about our budget and it wasn't rented.
So much to wander around to see in GTO. Taxis are cheap and met a driver to pick us up after 3rd nt (4:30 AM!!) to drive us to BJX. He even knew the house owner, Maria! Her house mgr usually gives tours but had knee problems, so she gave us a free "table tour" while we had drinks on the Plaza. Lots of little bars/cafes there.
The destination makes for a romantic or adventure trip for any age group. Not the best for small children. Make sure your legs are strong--I used alot of Aspercream and was begging for a hot bath after day 5. Good destination to not spend too much money, too! Best margaritas@ every bar...tequila and mezcal tasting for 2 of us. Even ate fried crickets (YUK!) and worm salt w/mezcal...Saw lots of shows on Plaza for Holy Week; even a free flamenco show around the corner from our house--outstanding!! Lovely churches and history everywhere. Don't miss the chance to visit if you can make it work.
Tripadvisor has some good posts regarding food and sights for both towns.
Adios!