The main difference is that the 638 comes with a percussion mode. The BOP is more powerful, more compact, lighter and more ergonomic than the 638. If you seldom (DIY) drill into stone i think it´s not necessary to have a percussion drill if you already own a "normal" drill. However there are more powerful percussion drills available from Bosch and Metabo with up to 1300 Watt and features which the Fein doesn´t have like the speed dial selector.
The Fein also has a plastic gear case, the others from Bosch/Metabo usually not.
But as a Fein fan why not buy the Fein
The "C" in the Dsceu Fein model number means that the drill is a hammer drill model.
An important thing to note with the Fein drills is that the hammer drill uses a threaded chuck attachment, whereas the non hammer rotary drill uses a tapered attachment. If you ever need to replace the chuck, either due to preferences or breakage, this difference could limit replacement options.
I have an older Fein hammer drill, It's very well made and works very well. I believe the front gear housing section may have actually been made by AEG since it looks similar to the gear housings on some of the older blue AEG drills. AEG was also known for their hammer drills.
I don't know about the current Fein hammer drills since Fein seems to have changed the gear housing, but the one I have is only plastic on the outside. The inside of the gear housing actually appears to be an aluminum or magnesium alloy with plastic molded over the outside, probably for insulation. My main gripe about this is that the 43mm collar is also over-molded with the plastic.
As far as wattage goes, Fein's tools routinely seem to have lower wattage than similar tools from competitors despite have similar or the same cutting capacity, and I've never had a problem using them to cut to capacity. The quality of the parts, design, engineering and assembly are likely more important than just the wattage specs.
Probably the best jigsaw available