Since I have never played vanilla for more than couple of days- my comparision will be:
a.MTW-VI-XL mod with Tiberius Expansion
b.RTW-BI-Europa Barbarorum1.1 with force diplomacy + sinhuet.

To encapsulate till recently I was playing (a.) but after a recent computer upgrade was forced to shift to (b.) due issues in MTW with the most recent drivers+hardware.

Simply put I prefer MTW-VI for the superb tactical ai and the semi-decent canpaign ai, but for an intense roleplaying campaign focussing more on the campaign map RTW is clearly superior due to the fact that traits and ancillaries are moddable and force diplomacy allows me to play a GA style campaign,so I can play a very defensive game with minimal expansion something not possible on MTW since I am forced to conquer a province in every reign to mantain decent influence(plus having 199 provinces helps). On the flip side battles are without any challenge for me even in Hard, even with the latest battle formation mods unlike MTW-VI.

Ps. my observations are based on very to moderately modded versions of the above mods- for ex my version of the XL mod has my own building mod which adds above 20 unique buildings featuring infinite build time and a RTR/EB style AOR system.