Both make sense. It depends on your experience / desire to try something new. Personally, if given a blank sheet of paper I think I'd go with R on R simply as it requires less coding. However, if time was a factor I'd go for PHP as I am experienced with it already. Also hosting is a factor - not all hosts yet support R / R on R.
I'd use PHP personally - but thats just because I love PHP and from experience know it can handle large-scale apps, and is very flexible/scalable.
Go with Ruby on Rails. RoR is built for scalability, those 'slow benchmarks' don't really mean anything to RoR especially since that stuff was going around a couple years ago during it's infancy. It's just getting stronger and stronger, and I'm betting it's heading towards the direction of becoming a dominant player more so than php.
DHH ( The creator of Rails ) posted about scalablitiy on his blog which might interest you, ( full post here) : Any language can scale if the site is designed properly.