The Oade battery is roughly $100 less with cable & charger than the EcoCharge system. They most likely use the same/similar SLA battery (7.2ah).
If you're ONLY going to run a V3 with it, then the Oade is probably more cost effective for you.
I run the EC-90 12V/6V for a couple of reasons.
1 - Gives me 6 & 12 Volt outputs, so that I can run a number of devices.
2 - I like the "charge indicator" on top of the the Ecocharge, so I can at least "ball park" the remaining charge.
For probably even less $$, you can go to
www.batteryweb.com (or any other battery source recommended on this board) and purchase a 12V/7.2Ah SLA. The correct switchcraft terminated cable has to be ordered (from Oade or Eco), or get the part number for the terminator and a couple'a bucks for connectors and you can make your own.
I like the features of the Eco-Charge products, and had $$ to blow when I bought them, so I went that route. Oade's should give you comperable performance without some of the bells & whistles, or you could build an Oade-Comparable system yourself...
Rick