Functional languages do have the advantage that most programmers that know them tend to be quite good, but I don't know if this is as true for Scala as it would be for Lisp, Haskell or Ocaml, as it seems that a lot of Scala programmers aren't in the functional camp, but more in the fed-up-with-Java one. And the quality of Java developers is all over the place…
However, it's less likely you could find a junior programmer or beginner to jump right into scala and 'get it'.
Keep in mind, if you go with RoR, you'll have no shortage of developers, but if you're project may encounter issues with scaling. Twitter has documented it's switch from ruby to scala for it's back end quite extensively, may be worth looking into.
Firstly, there is a huge established community to help you out and you can readily find programmers to hire.
Second, rails (or django) will allow you to get up and running quickly and will let you figure out whether your app is viable or not. If it works out, you can then start thinking about scaling and such. For now, use an established web framework(Django, Rails, Cakephp) and get it done.
it's completely possible to write the code in Scala that would be quite similar to the one you write in Ruby (especially JRuby in that particular case), although Scala allows a lot more, in terms of FP, threading and so on. if you're building a really high-tech startup, and your most important use cases are bound not only to front-end, and you need a reliable base for data processing, manipulation, hardcore server-side, use Scala. if it's going to be just a website with _some_ features that require a lot of processing, use RoR and maybe some Java/Scala for your backend processing.
I can't see any obvious benefits out of using Scala/Lift for mere website building against Ruby on Rails, unless you're making something i was talking in that topic...