A box
mod, in most cases, holds a removable and rechargable battery. There are a wide variety of designs and functions so it would be hard to classify which would be the best purchase for you. Eg.. Reo mods are beautiful, durable, fully mechanical and bottom feed but are not exactly inexpensive. Monkey Box mods are gorgeous and can be vari volt, some box mods are just that.. boxes to hold the components and
batteries.
A dual core cartomizer is designed to take a 3.7 volt ecig and give you the same effects as a 5v ecig. Use caution when purchasing a dcc to make sure that it is compatible with your ecig. Some disposable or semi disposable ecigs are not strong enough to use them and it can kill your battery.
Re: Riva and e-Power couldn't say as I've not tried e-Power.
My suggestion - decide what's important to you - aesthetics (must it look like an ecig?), battery life (if you are a somewhat heavy smoker this is important for you), versatility (can you use different vendor supplies for your ecig) and budget. Keep an open mind, look at all kinds of ecigs and mods and make a choice. Whatever the choice is that you make is better than smoking so just jump in and get started.
Hope that this helped and welcome to the ECF!!