Hi jim_tim
First you have to keep in mind, about this subject, 2 things: Normally it's not how good the render engine is that's important, it's how good the person who use it that's important; Second in the future, probably, the renders will be divided in two types only: real time (for a comparisson just see any high budget game) and unbiased (other types will mostly be for diferent looks like toon effects and such)...but don't worry because this will still take many years to hapen.
So what i can say to you about renders it's this:
Podium: Great capture of the skethcup easy to use and can produce some nice renders, but it's biggest strenght is it's biggest flaw: keeping things simple also means a big lack of depth in the render configuration, (any user that used 3D software before will reach its limits pretty fast) and because it runs inside sketchup, that right now it's everything but a stable and a up-to-date program, will probably crach in heavier models.
Indigo: This one is one to keep an eye on. The best part of indigo is not the render himself but the exporter skindigo, incredibly simple to learn and use and it's unbiased so it's pretty easy to produce photorealist renders but when you're running out of time that can be a problem. Other thing to keep in mind is that you can have problems exporting very large models if you're not on a 64bits OS.
Kerkythea: A complete render package. Can render both biased (for tigh deadlines) or unbiased (for more realism) and it's incredibly stable. You edit the materials outside sketchup (that be a plus for not relying on a unstable engine, or not if you want to do everything inside sketchup). You can also export a heavy model in parts and merge all in Kerkythea not needing this way a "big" PC for exporting.
Hypershot: A breath of fresh air in the render scene. It's incredibly fast and easy to use but for architectural work the images that produces it's still too CG. Where it really shines it's in industrial renders (metal stuff speccially, cars, objects...) and how it work's with hdris. If not the best, it's certanly one of the best in this department.
V-ray: Going right to the point, right now this is the best render that money can buy. Very fast, great renders and lot of options, but...and there's always a "but", it's anything but easy to master, even if you study the entire manual you'll still be very far away of what the real masters can do with it (there's classes where the main focus is rendering with v-ray...). One of the standarts in the advertising and movie industry.
Other's: Vue - great for landscapes, too CG for interiors; Artlantis - a fast render and setup but doesn't really have highlights any a specific area; Modo - I'm learning and trying this right now just because of the rendering so i can't coment now but it's one of the new rendering architectures that "everyone" it's talking right now and there's plans to release their "nexus" render engine to other Cad products.
So in my opinion kerkythea it's the way to go because its free, it adapts to your projects, and will start you in understanding materials configurations that can be aplied in other renders. If you want to make a carreer with 3D or have money and time to spent, THEN learn V-ray, because sooner or later you'll have a project that you'll need to use it or understand it at least.
Don't forget that the render it's just a small part of a great image, you also have: lightning the scene (one of the most important parts and where you'll probably lose more time with tests), materials (a good material can do miracles in a rough model), learn to random and dirty a scene (there's no perfection in real world) and a good 3d model (a sketchup output of a detailed model will probably be best than a render of a basic model). Just try the most renders you can and choose the best that suits you.
P.S.: Sorry for the long text and hope it helps
David