Some misinformation in this thread needs to be cleared up, the current Echo CS-490 and CS-500P were based off the Shindaiwa 446s which came out pre-merger back around 2007. It was a 46cc that was like all Shindaiwa saws were, rock solid, lightweight and quality built, but not exactly a screamer. I owned a 446s that I ported myself, turned it into a downright impressive machine. A 50cc Shindaiwa version called a 452s was in the works but never got released in the US before the 2009 merge with Echo. At the time, Shindaiwa had a chainsaw model 490, which was an updated version of their 488 and had nothing in common with the 446s or the current Echo CS-500P/CS-490. There was a couple years gap before Echo reintroduced the series as the 50cc CS-500P in 2011 and now the reduced cost CS-490. With the recent re-introduction of Shindaiwa branded chainsaws, the Echo CS-490 can be found (colored red and branded for Shindaiwa) in the Shindaiwa lineup as a CS-491.
Regarding the CS-490 vs CS-500P, I haven't gone over the IPL's in detail but other than the obvious and previously mentioned differences (handle, stock bar/chain), they do share the same piston & cylinder. They come with a very restrictive (but very quiet) muffler, so considerable gains can be had with a simple muffler mod.