This is a good one here. I just had an os speed, and I currently have a nova plus 4. I had and rb c6 but it was the older model, the first version of the bb. Out of the 3 the plus 4 is hands down my choice. The rb has great power, and had a great life span(over 10 gallons), but its only draw back was I did not get the great fule mileage every one claims.
On the os speed , very powerful motor, especially on the bottom end. To me in my mbx6 with a 2050 it was kind of a handful. The motor tunes great, idles great, netted pretty good runtime, about 9:00-9:30 mins for me. The biggest flaw to me though is the price! A speed with a 2050 will easily set you 5-6 bills. And from my previous vspecs I personally just could not get as many gallons out of the os as I could a novarossi.
The plus 4. This is my first one and I only have a gallon though the motor so far. The plus 4 is butter smooth, easier to drive for me. At only a gallon of fuel the motor already gets a little over 11 mins to a tank, idles and tunes great, and it is a nova mill so I expect the engine to easily go 10 gallons or so.
My vote is the plus 4.