Can speak so much for Rome Total War, and only a little on Medieval 2.
Rome Total War is plenty awesome, you take control of one of the three Roman Factions (Julii, Brutii and Scipii) each with their own beginning advantages (i.e. Julii can expand very quickly and usually only has to tend with the barbarian factions first, while Brutii can expand similarly quickly but has to tend with the more advanced Greek nations for example). Ultimately, while odds are heavily stacked in your favour (trade rights with three other factions on the onset, superior infantry) you will have the hardest task, overthrowing the Senate and your two rival factions. Only then can you complete the Campaign (while getting 50 territories under your fold), and begin playing as the other factions. As such, it gets tedious towards the end because your main opponents in the endgame are the Roman factions and it's just a slugfest of Legionary v Legionary. But, it is worth the effort to finish the Roman Campaign, because the non-Roman Factions are plenty awesome too
If you're feeling up to it, get the Barbarian Invasion expansion as well. It doesn't add on to the original campaign, but focuses more on the Late Roman Empire period. It is much harder to totally vanquish a faction (the entire civilian populace are converted to warriors and soldiers upon losing the last territory, becoming a neutral 'rebel' faction). IIRC, they add the Loyalty characteristic back too.
Appearance wise, Rome Total War is beautiful. The landscapes and battlefield scene are rich, and can even be destroyed by siege machines for morale
. Fighting scenes are not as great as Medieval II but at least you don't have 2d images running across the field like Medieval I. Music works perfectly with the pace of the battle (changing from peaceful to marching to fighting).
If there is one thing that irks me about Rome Total War, it is the AI. I am not aware of any AI mods that makes them fight properly
but sieges are awfully easy if your target only has wooden walls. I'll leave you to figure it out on your own, but the freebie I can drop is that the AI likes to move their units unnecessarily.
For Medieval II, mechanics are almost the same, and I am also aware that the Christian factions have the Pope to contend with if they want to expand. Since I've not played the full campaign yet I'll reserve my comments on its difficulty. Also, you should be able to get all available factions from the beginning.
However, I am aware of a Mod (Stainless Steel, iirc) that increases the AI's capabilities, and also includes new factions. Interaction between units on the battlefield are also smoother and more seamless.