Having now been exposed to both...
If you ever want to use it as a caching system ... solr is the way to go.
I personally think very highly of Sphinx, yet be aware that Sphinx will return ids not "content".
When handlesocket was announced [1], for me the gem other than the obvious was this nugget of information.
"No duplicate cache When you use memcached to cache MySQL/InnoDB records, records are cached in both memcached and InnoDB buffer pool. They are duplicate so less efficient (Memory is still expensive!). Since HandlerSocket plugin accesses to InnoDB storage engine, records are cached inside InnoDB buffer pool, which can be reused by other SQL statements."
Knowing that with the latest version of sphinx your can make use of their Query format via code or via the mysql console... it has me more sold on Sphinx than solr.
We use solr where I work, it is nice to use and coming features will make it better...
The most annoying aspect of solr for me is, you cant index elsewhere and easily merge it...
Either are miles better than fulltext search.
Perhaps list a few of your needs and requirements then we might be able to see if one stands out ahead of the other.
[1]http://yoshinorimatsunobu.blogspot.com/2010/10/using-mysql-a...