Both!
(Well I think that's a totally reasonable idea..
)
Seriously though, if you have the time, why not do both? I understand that you might not have the time though, and if you plan on doing stories it can get really hectic to play them all and get them updated at a reasonable rate as well. (No one mention that my immortal dynasty was supposed to be updated today..) Personally, I started with the Immortal Dynasty first because it's the classic one that everyone wants to complete. I started that with the intention of moving onto the DecaDynasty once I had completed it but it just drew me in a lot earlier and I felt that I wanted another file to play with.
I'm finding the DecaDynasty a lot more fun because I actually feel there's more freedom. I'm not really sure why, probaby because there's the option to start with a rich sim which I have chosen to do. (You won't always be rich though, unless you employ the Storing-Stuff-In-Chests-Outside method which a few DecaDynasty players have been doing). My immortal dynasty just bores me at the moment, because I'm at that stage where nothing happens.. Not sure if that stage ever comes in the DecaDynasty too, but my first heir is only a toddler so there hasn't really been enough time for me to get bored.
It's personal preference. I think they're as challenging as each other, though I find the Immortal Dynasty more intimidating despite talk of the DecaDynasty being a lot harder.
If you're someone that finds yourself getting bored easily with files, go for the DecaDynasty because I suspect that moving houses around all the time will keep your interest - I know it'll keep mine.