If you already have an isolator then you only need a single output charger not dual. It would be a good idea to consider replacing your isolator with an ACR or automatic combining relay as there is no voltage drop through diodes like you get with a diode based isolator. ACR's are about $70.00-80.00 +/- depending upon where you buy it: http://www.defender.com/product.jsp?path=-1|328|51495|299265&id=750182

An Echo Charger is also an excellent way to keep the start bank topped up but admittedly more expensive than an ACR: http://www.defender.com/product.jsp?path=-1|328|985|316473&id=93959

The gust 6 amp model with single output is only $65.99 at Defender: http://www.defender.com/product.jsp?path=-1|328|985|316471&id=131403 and if you're tied to a dock then the speed of charging is not really all that important. Keep in mind that the dual output Guest chargers are 5A & 5A each leg for a total of 10A. If you used both outputs and you start bank was full, as it normally is, you'd still only put out 5A to the house bank so a 6A charger can actually get you to about the same net output using an ACR.

While I tend to agree with much of what has been said about Xantrex they have changed manufacturers on many of their products and a lot of the new stuff is a much better than it has been. For example their battery monitors are no longer the buggy "Link" series originally designed by Cruising Equipment Co. which then became Heart Interface and then Xantrex. Their new "Link" battery monitors, Link-Lite & Link-Pro are now made by TBS Electronics in the Netherlands. TBS builds products for Xantrex and Victron yet Victron is very well regarded? Same shared behind the scenes manufacturer for some products...

Some of their newer battery chargers have been a lot more reliable than the older stuff but you really don't need anything that expensive for what you want to do. As much as I dislike SOME of Xantrex's past products and their past support I have not thrown the baby out with the bath water as they do still make some excellent & reliable products. I have installed a slew of Xantrex Echo Chargers and never had even one failure so it just depends upon which Xantrex product you get..

Years ago I swore I would never again buy Raymarine because their support was utterly horrendous. Today I feel they now have some of the best customer support in the industry... Their products are still not as reliable as I'd like but they stand behind it, support it and have great turn-around.