I dunno, both are pretty darn tough to make, but in very different ways. I'd say beat 'em up is better
Disclaimer: Any sarcasm in my posts will not be mentioned as that would ruin the purpose. It is assumed that the reader is intelligent enough to tell the difference between what is sarcasm and what is not.
Beat-em-ups are more fun. RPGs are also going to take longer to make (if you want a good one), cause the good ones are always epicly huge in scope, taking months to finish playing, and depend heavily upon a good storyline.
Quite honestly, I'd rather read a good book than to waste my time with an RPG. With a RPG, I have to fight endless meaningless battles where I don't have full control over the character before I get to see what happens next in the story. With a book I just smoothly read through the words and let my imagination do the rest.
If you want to make a truly balanced and fun to play beat-em-up, then take a look at King of Dragons for SNES, and make a similar style.
It's also probably the easiest style to make since you only have the one attack that gains in power and ability, and a jump.
There is a magic attack too that I rarely use. Yea, I still play King of Dragons even though I beat it on the highest difficulty (just with the elf tho). If you learn all the patterns and tricks, you can actually beat the game without getting hit. It's a tough game to even beat though, that's why I say it's such a balanced game, it can be beat without taking damage, but good luck trying.
Blood of the Ancient One, Seen only as Shadow, Faster than Lightning, Fierce as the Greatest Dragon, Nearly Invisible, Floating in a Dream, Entered through the Demon Door, Destroyer of Evil in a Realm with a Red Sky Scarred, Who could I be ?
I had fun playing Naruto: Uzumaki Chronicles 2 for the ps2 with my sister. You play a made up story based on some puppet soldiers causing some trouble with the odd non main character joining in based on the story and a second player can always join in. You gain EXP and money from the enemies and can level up any of the characters as well as buying upgrades for them.
There is also a mission mode to play but they're usually something like beating up 20 thieves in 2 minutes or something lame.
Also the camera with 2 players is the worst thing I've ever seen, its horrible.
But it's still surprisingly fun especially spamming people with hundreds of naruto's shadow clones. (and in turn annoying my sister who wants all of the money)