I agree. The L5630 may be a few generations later than the L5420 (in this case the numbers are in sequence), but you'd likely have a hard time benchmarking much of a difference between the two in the VPS environment. Even if there was a specific benchmark that could show a clear difference (say, due to the different cache structure) I suspect the behavior would get dwarfed by impact from other guests on the same host.
Not that it isn't a logical question to ask, if only for curiosity's sake. Probably the only way to know for sure is to come up with a metric that makes sense for your application and test the two cases. That way you're also taking into account the entire environment (other guest load, I/O contention, etc...) Probably not worth the effort unless you're seeing performance issues.
You might find
http://en.wikipedia.org/wiki/Xeon helpful for at least placing the processors you might find on a Linode into the right families.
-- David