The question you should ask when choosing between RAM and SSD is, "How much does my application really need to use the disk?"
If you have an application that is mostly scripts doing computation and sending custom versions of pages to users and handling the same kind of requests over not very much data over and over again, then RAM is what you need.
If, however, you are running a file server, streaming media, or thrashing random files that won't fit in RAM, the extra speed given by an SSD will help you a lot. Another place you'll see a good payoff for SSDs is if the application does a lot of writes. SSDs typically have much better read than write performance, but they are still much faster than platter drives.
Where I work, we have an application that writes several GB of data over tens of thousands of RRDs every few minutes. Upgrading the server to one with a fast SSD improved performance dramatically. The same probably could not be said for the server that handles, say, the authentication database. Although that database is large, queries on it are frequently similar (same people logging into the same apps over and over), so caching in RAM is much more beneficial.
Basically, if you're serving the same requests over and over, go for RAM. The OS will cache the data you use frequently and you'll see good performance. If you're application does a lot of IO to "random" files, you'll be thrashing the cache and will see more benefit from a fast SSD.