Yes it will be for arch viz. Don't need any advanced interactions.
I guess I could just try out lumion and compare the process. I just find there's a ton of stuff in udk that's randomly difficult and arbitrary or non-intuitive.
Terrain tools aren't too important if you can just import a decent mesh I suppose. I wonder though, does lumion have a kind of geometry painter for grass and plants?
I have some experience on both of them so I can give you my opinion:
UDK has the advantage of having a huge community behind, and it is one of the most used game engine around...good features, you can do whatever you want if you know how to do that...
Basically the stongest features of UDK are customization, interactivity, strong visuals and post process effects, material creation is almost perfect...
Lumion is straight forward, intuitive UI, fast DAE/FBX import, fast materials setup, lighting looks very good, perfect to prototype and make HD movies
Now if you're looking for an Architectural Visualization Tool, just go with Lumion...a couple of reasons:
Require mesh optimization ( low poly ) and lots of normal maps
Learning curve could take quite a bit
Lighting builds could take a lot of time ( lightmaps with GI )
If you don't know how to program or you're not into game engine, it could take ages to get an .exe working properly
It is in general more time consuming
Content available only suitable for games
Possibility to import meshes without any optimization ( someone posted a stage with 18+milions polys )
Material setup fast, good but not that customizable
Useful content perfect for ArchViz
The preview is almost what you get in realtime
HD Movies in a reasonable amount of time, still renders very good
If you're looking for an ArchViz just go with Lumion, UDK is something that you would use only if your project is focused on some elements, like interactivity, realtime walking into a stage, or some specific feature that needs to be coded.
Hope it helps