I have a Maruzen MP5-k (original model) and a KSC TMP both are a blast.
Maruzen MP5-k
Pro's
One of the highest ROF's for an un-upgraded gun around.
Mags/Accessories are easy to find (now that it's back into production, that wasn't the case 6mos ago)
Nice Solid Feel
Functional charging handle. Some people question it's reliability, but I have performed the "HK Slap" countless times on it.
Con's
134a only, Green Gas will damage the gun
Included short mag is worthless. One burst will empty the mag. You will want the full length mag.
Mags are stupidly expensive (compared to an AEG) and to use this as a primary you will need a lot of them.
KSC TMP
Pro's
Shoots harder then the MP5-k
Shoots louder then the MP5-k (I like that in my gas guns)
Extremely uncommon weapon on the field
Uses Green Gas out of the box
Con's
Newer models require a slight tweak to prevent it from destroying itself.
You have to be gentle with the mag or risk breaking the nozzle.
Essentially 0 accessories available.
Mags can be extremely difficult to find.
Since they have started producing the K again, I would say pick one up. I have pondered getting another one just because I can...
However, keep in mind the mag cost and if they stop production again accessories and mags will quickly become hard to find.
If you have a place to meet, I would be happy to show you mind so you can get a feel for one before you buy.