replacing obsolete HEMT and differences between Package/Case and Supplier Device Package
Component package designations are a situation where you have several different partly overlapping "standard" conventions, and then a bunch of manufacturers who have created their own schemes.  

The "SC" designation is a JEDEC convention, the SOT designation is, I believe, IPC, and there are a handful of other pseudo-standards that have cropped up to cover various configurations that don't exactly match the standards.  Many manufacturers have their own more tightly-standardized packages, with their own designations. 

Even when two devices claim the exact same designation, the exact dimensions may differ slightly or have different tolerances, so it's important to always check the drawing in the datasheet to verify that the dimensions are close enough.  If you've designed a PCB footprint for a device that is on the small side of the standard range, then you may wind up with insufficient fillets if you later switch to a device that is on the larger side.  0.65mm SOT parts are small enough that the tolerances DO matter here.

The only time you can really be sure that a device is going to be in exactly the same package as another is if it's from the same manufacturer and references the exact same manufacturer's drawing.



Regarding the part characteristics, don't rely on what Digikey has chosen to list in their catalog.  It's very difficult to distill a device's performance down to a handful of numbers.  The datasheet probably lists several different values for each of the parameters that Digikey has chosen to list, at different operating points.  In order to compare a single parameter between parts--say, the output power--you need to ensure that those parameters were measured under equivalent conditions.  Consider the catalog numbers to be a very rough guideline, and dig into the datasheet to make sure that the device's performance will meet your needs under the conditions in which you intend to use it.