This is really the crux of the argument: What do you want to have when you finish?
If you just want the completed application ASAP? Go with what you know, and don't waste time on learning anything new.
Want a completed and familiarity with a new language + framework, at the cost of delayed launch? Go with the new language.
The application you've described is simple enough to be done with any framework + language fairly quickly, so no worries there. Just weigh your motivations and get to work!
On a side note: I'd go for rails (ruby), then django (python), then symfony (PHP) by personal choice. The generators for all of these languages will save quite some time, likely.
If you just want the completed application ASAP? Go with what you know, and don't waste time on learning anything new.
Want a completed and familiarity with a new language + framework, at the cost of delayed launch? Go with the new language.
The application you've described is simple enough to be done with any framework + language fairly quickly, so no worries there. Just weigh your motivations and get to work!
On a side note: I'd go for rails (ruby), then django (python), then symfony (PHP) by personal choice. The generators for all of these languages will save quite some time, likely.