I have a Beretta Storm in 9mm and a Scorpion. Each has some advantages and disadvantages. Both have been utterly reliable for me but I have ten times as many rounds through the Storm (had it a lot longer). Neither will use Glock mags if that's a deal breaker. Here's my take.
Storm- Pros:
- Very short OAL (shorter than my AR with the stock fully collapsed)
- Extremely reliable
- Totally Ambi
- Mags are rock solid, reliable and dirt cheap (from 10 round up to 30 round) with the 92 version
- Full top rail
- Great handling and ergos
Storm- Cons;
- Mediocre sights that aren't removable
- Lots of proprietary parts
- Miserable trigger
- Crossbolt safety is awkward for me (but it could be a pro if you're used to that)
Scorpion- Pros:
- Handling is "modern" with controls similar to other military-type weapons
- Great ergos
- Good (removable) sights, full top rail
- Extremely reliable
- Good practical accuracy
Scorpion- Cons:
- Mags are a little iffy compared to the steel mags of the Beretta
- Mags are proprietary
- Needs the safety delete kit or it's hard on your shooting hand
- Trigger is better than the Storm but not great (good aftermarket support though)
- A little more 922r work involved
- Very large and heavy for a 9mm
Overall I think the Scorpion is a little better design, or at least it seems a bit more intuitive to me. If you like the MP5 you'll find the layout pretty familiar. The folding stock is great and everything is thoughtfully laid out. I do think it's a bit absurd that the gun is so heavy but the massive bolt adds a lot of weight. There seem to reports of the mags splitting at the feed lips. I haven't experienced with but I've only had the gun for a year. It's a propriety mag, unlike the cheap and ubiquitous Beretta 92F mags. Again, I have had no issues but the steel 92 mags are very reassuring in their robustness.
The Storm seems to have been designed to appear more "sporting" and non-military although it's been used by militaries. The bolt release is kind of weird and the cross bolt safety still doesn't feel intuitive to me. If you run a Remington 870 though you'd be right at home. Triggers seem to vary from specimen to specimen but mine is miserable. Simply awful. Even with thousands of rounds through it I occasionally think the safety is on; that's how heavy the trigger is. However, it's still very easy to shoot well. The sights are not great and can't easily be removed (although they do fold down flat out of the way). I really wish the gun came with a slick flat top rail (like the Sierra Papa conversion) or like an AR.
Both of mine get used as range toys but the Beretta is also one of my three main HD/zombie guns. Eventually I'll get the Scorpion set up in that role but it's not there yet. The main virtue of the Storm is the extremely short OAL. Seriously, it's about as short as a gun can legally be. It's shorter than my AR with the stock fully collapsed. For indoor/defense/CQB use it's extremely easy to maneuver through doorways, around corners, etc.
Blowback guns are generally pretty simple and reliable. Unlike a blowback AR there's no messing around with buffers, etc to mod the gun to work right. The Storm and the Scorpion run like a raped ape right out of the box. Maybe the slight edge goes to the Storm with the 92 F mags but the jury is probably still out on that.
If you want to tacticool it up the Scorpion is probably the better gun. It has M-LOK handguards and will be very familiar compared to common assault-type rifles and subguns. If you want it to be kept relatively clean and sleek with a minimum of crap hanging off it the Storm works great. Mine has a RDS, a white light, VFG and a sling and that works well.
If you can get the tax stamp then it's probably no contest- get the pistol version of the Scorpion, the 922r kit and the stock. Rock and roll!
I can't see you going wrong with either gun for plinking or home defense.