I own a CA M249 Para and can tell you that without a doubt in my mind, it is the most durable, skirmish-worthy, most realistic stock AEG on the market.
The springs can be changed in literally 20 seconds for a higher/lower fps, it is made completely of metal excpet for the grips (like the real thing), and is compatible with standard Marui type upgrade parts.
I am very familiar with the real 249, and find this to be a very true replica.
I popped in an M120 and Hurricane FTK and I was shooting 412fps with .20g BB and the HOP UP off.
With a PDI 170 and .20g BBs I was right below 400. With .25s and HOPUP set for a flat trajectory, I was around 370fps.
The CA para is compatible with the aftermarket RAS units (I'd use a real Knights 249 RAS, but it's not compatible with thr airsoft gun 249). I bought a G&G M249 RAS from AEX for little over 80 and it installed in less than 5 minutes.
I've heard that the MkII from CA must be modified to use the G&G and G&P RAS.
I also added a custom modified M4 stock. I run a First Factory/ANGS AN/PEQ2A out front on the RAS with a 1700mAh 9.6v pack. I also use a vertical foregrip at the 6 o'clock rail on the RAS.
I have no experience whatsoever with the STAR SAWs, but I have extensive experience with plastic AEGs and I can tell you that, cost aside, it is not worth buying. Plastic mechbox, plastic body, and other plastic parts will make this a nice starter support weapon, but it just will not stand up to the abuse that the CA SAWs will.
Remember when you first get your CA SAW to take out all the screws on the reciever, degrease them, and reinstall them with some Loctite. A few of them worked their way out near the carry handle, but after a bit of Preventative Maintenance with loctite, they haven't bothered me after 3 games and several thousand rounds.
I also run the CA box mag and it works great. Kind of costly for what it is, so I suggest taking a look at the Guarder plans for a 249 mag available on their website as a PDF. If you don't think you can pull it off, buy the pre-made unit. Myself? I wish I had waited until after I saw those plans.
If you have any questions, let me know. If you'd like to take a look at it and testfire it, I'll be at the game this Saturday.