I'll jump in with my two cents.
I use Thea at work (architecture office) and Kerkythea at home (free). They both work very well/easily with sketchup and either would work just fine for you. If you don't mind paying a bit Thea is the better program (no surprise) but you might not need it.
There are plenty of differences between the two (active development not least) but the two main items I notice switching between the two are displacement and layers. Of these, layers is BY FAR the more important. Displacement is nice, and occasionally I do use it, but it depends on the type of result you're looking for.
Professionally, the images I'm trying to get are meant to show a compelling presentation of the proposed design to the city or a client. Reasonably believable materials and colors are important, as are good shadows (especially to show depth). Either program will do that just fine. Displacement makes for longer render times and more setup. Plus, if they're overdone they look as bad as if they weren't used at all. There are some real artists who use it in their renderings but are you showing closeups that honestly necessitate that detail (possibly) and do you want to spend the extra time required? For me it's rarely worth the time.
I tend to like "pretty good" renderings, that specifically establish the desired/required lighting and shadows, and then finish in post-pro (these are some great tutorials for that). It's faster and gives you good looking results, even if it's not PR.
About layers. When models start getting very large (usually when doing detailed exterior scenes) it's really nice to be able to turn off some variation of: the grass, the cars, the trees, the people, etc. It's such a basic feature but unless I've missed it you can't do that with Kerkythea.
Instead of displacing grass, I usually either use instances (very easy to do in either program) or post-pro as in the tutorial listed above.
Basically, get Thea if the expense makes sense for your business. Since you're just getting it, I imagine you've been doing fine without it in the past and this is just a new (very helpful) area. If that's the case, kerkythea is the way to go; if it works well then upgrade and the workflow if very similar.
Longer post than I intended. Sorry.