Building trees, family trees, historical accuracy, probably even balance and AI are things that speak for M2 rather than rome. Rome has better graphics ofc. And naval combat, which probably doesnt happen that often since navy is of little importance in both games I'd say.

I would either suggest Attila (which is basically an improved Rome2) or M2, since you mentioned stainless steel I would shamelessly advertise a submod of my own for it which basically overhauls everything again:

http://www.twcenter.net/forums/forumdisplay.php?2055-Stainless-Steel-Historical-Improvement-Project-(SSHIP)


To give you an illustration, we use models of this little mod which makes units still look pretty awesome for a 15 year old game:

http://www.twcenter.net/forums/showthread.php?640851-Western-Unit-Overhaul-v4-0-(Jan-26-2015)



Other huge improvement mods to vanilla are Deus Lo Vult, which is more character/roleplay oriented than Stainless Steel:

www.twcenter.net/forums/forumdisplay.php?309-Deus-Lo-Vult


And if you want to shift the era to the roman period rather than the medieval, europa barbaorum is your mod:

http://www.twcenter.net/forums/forumdisplay.php?454-Europa-Barbarorum-II