I use both Vue Xstream as a stand-alone and also in conjunction with C4D. I also use Carrara quite heavily. 

The are both excellent solutions but there are some differences. Vue's ecosystem will take a single tree instance and then alter it during the replication procedure so branch positions and shape are altered. Carrara does not do that but  you can randomize scale and rotation which significantly hides replication issues - and mixing an additional tree or to in to the replicator helps further. Carrara's replication system is faster to manage/adjust content in than Vue's.

Carrara has an a landscape mapping scheme so you can control by altitude, slope and other features as in Vue. Carrara does not have the Ecosystem capabilities to automatically reduce plant replications in shaded areas or near specified objects This can be done by using a custom map which you can paint from within Carrara if you want - but it's a manual thing. Vue lets you "paint" objects right into your scene but Carrara requires you paint on a map and then update the replication. 

Martin (aka Howie Farkes) has done a brilliant job within Carrara of using maps to control placement - but I see he also has the same geometry for sale and use within Vue's Ecosystem now. 

They both have about the same learning curve regarding creating landscapes. Vue has better atmospheric controls than Carrara. You can't actually create anything with Vue except via the Bryce-ish primitive boolean stuff. Carrara has a full bore modeler and an easier-to-use texture scheme and works much better with DAZ/Poser content. Animation is easier to do in Carrara. Vue has animation presets that Carrara does not. 

As with Vue, the quality of the Carrara results is dependent on the user to a very large degree. I'd created this image during the testing of Carrara's replication system when in conjunction with the (then) new transluscency capabilities when it first came out 4 years ago. Howie does much more sophisticated work than I in Carrara landscapes but getting good results really just takes learning the software - same as always! ;-)

Just the made ravings of an insane 3D user...

Mark