"A dyno is roughly equivalent to a Mongrel, except that
dynos are spread across multiple servers, so
performance in most cases is greater.
4 dynos are equivalent to the compute power of one
CPU-core on other systems."
A dyno is different from the common RAM+Bandwidth VPS slice, so it's difficult to make a comparison. (I'm going to do it anyway...) I would say that a 4 dyno machine, with everything else dialed down to the lowest value (so, free), you're roughly equivalent to Slicehost's 1024MB slice. (That's roughly $108/mo at Heroku, $70 at Slicehost.) You've got more storage space (so, the db can grow) on Slicehost, but you have to setup everything yourself, and adding power will take longer than a couple seconds.
It should be stressed that Heroku makes everything really simple. You can save a lot of time and energy if you've got the money to get started with Heroku up front.
I like setting up and tweaking a VPS, though. I haven't seen any benchmarks to show that Heroku with 4 dynos performs as well or better than 4 mongrels, or Apache + mod_rails on the 1024 slice, etc.
I think it's a bit expensive, but, man, it's really cool, too.
I'd argue that a Dyno isn't nearly as resource intensive. If 4 Dynos == 1024MB slice, then each Dyno would be using 256MB. Ruby app servers don't need that much RAM each and with a 1024MB slice you could fit a good deal more than 4 Dynos in it.
In fact, the Phusion Passenger documentation says that they recommend 30 app servers within 2GB of RAM. They do say that if you're running lots of other things like MySQL on a 256MB VPS, you probably want to limit yourself to two app servers. Still, it scales up much nicer. For $70/mo you could easily fit 10 app servers with your database on the same box - something that would cost several hundred dollars with Heroku.
That said, Heroku's selling point won't be cost. Heroku makes deployment exceedingly simple. You don't have to worry about installing anything, configuring anything, making sure things stay up, etc. And that has value - especially if you aren't a *nix gearhead. It's more than a slight premium, but it can still be worth it given that it gives you a worry-free environment.
It should be stressed that Heroku makes everything really simple. You can save a lot of time and energy if you've got the money to get started with Heroku up front.
I like setting up and tweaking a VPS, though. I haven't seen any benchmarks to show that Heroku with 4 dynos performs as well or better than 4 mongrels, or Apache + mod_rails on the 1024 slice, etc.
I think it's a bit expensive, but, man, it's really cool, too.