Crimson Trace Lasergrip - LG 639 vs 619

[Captains1911]

In comparing these two models for Glock 19/23 Gen3, the only differences I see is that the 639 has an on/off switch, and is shaped a little differently. Is this correct? Any reason not to go with the 639 other than the slightly higher price?

Also, do these work with most standard holsters, primarily hybrid IWB style? Thanks.

http://www.crimsontrace.com/products/manufacturer/glock/01-6910

http://www.crimsontrace.com/products/manufacturer/glock/01-1330