The 649 up to the -2 models were .38 special. With the -3 model the frame was changed to the "J-Magnum" frame to accomodate the .357 Magnum cartridge.
The .38 special version will weigh in at 20oz versus 23oz for the .357. The 642 is an
Airweight gun and only weighs 15.8 oz.
You'll find that the .38 frame and barrel are a tad shorter. The .38's have a 2" barrel while the .357 is 2 1/8" long. (There were some 649-3's in .38 special made in 97-98 on the new frame with 1 7/8" barrels - your $329 model seems to be one of these).
Supica's 2001 edition of the Standard Catalog of S&W lists a 649 .38 special between $250 in VG condition to $370 NIB. Your deal of around $300 is decent if the gun is in Excellent condition.
For a size & shape comparision, see the following image:
S&W Model 042 (top)
Airweight .38 Special and S&W Model 649-3 .357 Magnum (bottom). The Model 042 is was a single-year transition model between the older Model 42 and the .357 642.