Tripower is nice due to the fact that you do not need a battery. It is super bright just being powered by sunlight or whatever light you have. In total darkness, it has the tritium. When you are in shade and do not have bright light above you, yet you are looking out toward a very brightly lit area - you hit the button and it uses the battery to illuminate eliminating the dreaded "washout" that MOST red dots suffer from (except EOtech and Aimpoint). Trijicon also has superior customer service over Aimpoint (dunno about EOtech). Trijicon will take care of you fast. Aimpoint is difficult to get a hold of, you usually get a machine, and they get back to you. Most fixes require shipping back to Sweden taking at least a couple months. Fortunately, they are one of the best and most never need any service ever.
Honestly, the absolute best price/performance red dot out there is the Aimpoint Comp C3. It's the same body and design as the M2 and M3 series, but it is a civilian version. What makes it civilian? Well, for one, it is rated at 17ft waterproof, vs. 75ft or more for the others. It doesn't come with flip up covers, nor does it have the adjustment cover tethers/lanyards. Compared to the M3, it also doesn't have the outer rubber body sleeve. Although, they sell these separately I've seen.
For all practical purposes, other than Navy SEAL use (scuba diving with it), it is just as tough as the others as it is the same thing. It has the same dot, same knob. Same 50,000 hour battery life. But you save at least $100 or more going with that. It should be perfectly fine for any kind of "combat" a civilian would see, whether it would be defense or TEOTWAWKI. It might even be useful for night vision devices as the first 3 settings are nearly invisible just like on the NV models, but I would NOT quote me on that - it may lack special coatings. Some people have said they had no problems with NV, however, that is internet hearsay.
If you want something more compact, look into the new micros by Aimpoint.
Going on an M1A, you will be pleased to find that the standard Aimpoint ring (comes with it) is the perfect method for mounting it (lowest). Whereas guys with AR-15's need to mount it higher adding costs by having to buy an expensive mount.