I perfer the eGo button, but the riva's 750mHa SEEMS to give a more consistent voltage vaping, honestly as time goes on your going to try all kinds of batteries least I did, 650's, 750's, 900's and 1100's, by a variety of manufacturers. Joye does make a 750, which I didn't like but I loved the 900.
Now the long and short of what I'm trying to say is, if this is your first e-cig, you can't go wrong with EITHER choice, because this is just the start of hopefully a long but happy journey for you and your e-cig. Once you start "saving" a few dollars on analogs you'll start trying all sorts of different things to find your "magic spot" and no one but you can really find that, because we all want different things out of this.
For me I started with the Joye eGo, tried ALL the cart mods out there, ended up doing standard drip instead of the mods. Tried all sorts of batteries to enhance my experience. Tried bottom feeders etc. This was almost a year ago I started my journey and I was "afraid" of making a mistake, and well I did make a mistake I tried the Blu before I found this forum, but after asking like you did, I made the jump and havent looked back!
GOOD LUCK!