I'd go with Carbonite, now that it is free. It's a much more useful and powerful addon than QuestHelper, and more efficiently coded as well.
As far as taking over your UI, it doesn't unless you turn on the options for it to do so (it can replace your minimap if you choose to, but you can also have it a secondary window that you can turn off/on at well). It does replace your main map, but it's a vastly superior map anyway, so I don't see the problem with that.
It has numerous other features that are useful, mostly to do with map tagging. Tagging quest givers, resource nodes, flight masters, inns, etc on your map. All filterable of course. Notification when an enemy is nearby, or when an ally running Carbonite is as well (so if you see a friend doing the same quest as you, you can team up). The Warehouse is nice if you don't use inventory mods (it shows you items that all your characters collectively have).
As far as the quest help that it gives, it colours in areas on the map that the quest can be completed at (So if it's a mob that spawns, it shows the entire area that it can spawn within, highlighted).
Carbonite is a lot more than just a quest addon, but it can be as much or as little as you want, it's highly configurable. One of the best made addons I've come across (likely due to it being paid for a while).