I don't know a ton about the e-power. I know I own an ego and it's not failed on me. It's a tried and tested workhorse of the
vaping world. The riva is the same thing but it's regulated at a slightly higher voltage.
I like the idea of the epower but I'm skeptical about reccommending it because it's new. I've seen some posts about problems with the buttons falling out and things not fitting it quite right. I don't mean it's bad or anthing, I don't know enough about it. Because I already own an ego, I don't need a riva or an epower but who's to say it's not going to work for you.
Simply based on a tried and tested ecig that should be very robust, I'd say take the riva, get some cartomisers for it and give it a blast. Biggest plus is the battery life. It'll last a lot longer than your 510 and it's not massively bigger. The epower is a fair bit longer and a bit wider I think. It's also more convenient to me to just have the battery screw in to my laptop and charge instead of having to find a wall socket to charge a battery with after I took the ecig apart to take it out.
Right now, I wouldn't get an epower but only because I have an ego already so I have no need for one. Just my two cents but it's up to you.
It's not about form really either. I also own a box mod
http://www.liberty-flights.co.uk/product.asp?id=59 the tank box mod. It's a little similar to how the epower is. Removable battery but it's a little flat box and looks hilariously like a detonator switch. It's a little less convenient to carry and recharge the batteries but it performs like an ego so I consider it a backup to use at home.
Edit: oh wait, isn't the epower an auto battery? I'd find that really annoying with a big pv, I'd be too tempted to leave it hanging out my mouth like I do with my volt

I like autos for that but wouldn't want it on something that big.