My favorite Mexican in the Northville area is Border Cantina on Novi Rd about 1/2 mile north of Eight Mile.
There's also an On the Border on Haggerty Rd. just north of 8 Mile.
If you change your mind, there's a mongolian bbq type restaurant called Sizzling Sticks that's just behind the band shell. They also serve up some really good salsa and chips with your meal. You'd probably have to get there early though because of where they're located.
I like Border Cantina a lot, but the service is sometimes spotty and the wait on the weekends can be mindblowing. You get a lot of food for the price. Fajitas and chicken nachos are especially good.
In Livonia on 8 and Grand River is El Nibble Nook. A little divey (and much reviled on Chowhound) but I have never had a bad meal there and it is consistently filled with locals who have eaten there for years, so they must be doing at least something right. Good margaritas. Absolutely must try the Tijuana Bypass.