That is a tough call. You will like the sportsbook better at the M but, there are more food options at Southpoint and I think the slots pay a little better there plus you will find more lower minimum tables.
Honestly, I would take the best room deal and be confident you will have a great experience. Either way, make sure your wife signs up for the slots club immediately. We had our entire stay comped at the South Point based on my wife's play plus she won money. (mainly 3-hand VP for $3.75 a roll for hours on end)
At South Point, someone already mentioned the prime rib dinner which is a great buy. They have a Steak and Shake there as well as a movie theather. A bonus for you could be the spa is excellent. The buffet is not bad and very low priced. The deli next to the sports book is probably the best in LV.
When I was there last (Dec) they had a tequila bar right next to the Mexican restaurant. All shots and tequila drinks were $2 including Patron.
Racebook is full of old timers (it is separate from the sports book). They do have more betting options at the sports book and the waiting line is never too long.
Rooms are much nicer than any standard room at a Harrahs property but, M has the edge there. You are about four miles closer to the strip at SP and they will arrange to pick you up at the airport if you call ahead (also drop you off...both free).
If you like the racebook, I would plan a day, go to the M eat the buffet for lunch or dinner and spend the day in the book (sports and racing together). I think a cab ride would be well under $20.
Make sure your wife gets an M card too and you should since they take it at the racebook. If you stay there, I can give you the name of my host and, after a few days there, give him a call and he will evaluate your play and see what he can do.