The Humble Origin Bundle has Command and Conquer Generals with the Total War expansion for $1.Alternatively, for $20 you can get Rise of Nations: Extended Edition.Rise of Nations Gold is one of my favorite RTS's ever, and from what I've heard, this updated version is even better.
C&CG is $1 (and you get a few other games). From what I've played, matches can be around 30 minutes to an hour long, though it may be long if you're fighting AI. You also start right off in the modern day. You will also need a workaround to play online multiplayer.
Rise of Nations is a bit more long term, and is also more expensive. In RoN, you work your nation up from the ancient ages to our modern age - and the future. There is a lot more management at hand, such as resources, tech trees, diplomacy, etc. The steam version is also updated so it doesn't rely on gamespy, and it has ranked multiplayer matches. You'll also be able to manage larger armies than in C&CG. There's also a singleplayer campaign that's similar to Risk, but in real time.
Overall, it's all up to preference and what you're looking for. C&CG is a bit more of a military strategy game than RoN, but you don't have to go through all of the ages in RoN. You can start in any age you want, so if you choose the information age, you'll be able to have modern military right off the bat.