The CIRAS is full wrap around armor and as such maybe slightly bulkier/restrictive, but offers more MOLLE real estate.
The RRV by itself can be configured several ways (chest rig with or without hard plate, straight or crossed shoulder straps, or with a real panel attached), making it a bit more versatile. The typical MFR setup is with the rear panel (usually with a MAP) attached -- providing front and rear hard plates -- and worn over a PACA LowVis soft armor.
You lose MOLLE space on the sides but the setup feels a bit less restrictive: it's similar to the MBSS+PACA setup but with the RRV offering more MOLLE space.
PANTAC's CIRAS is generally considered a very good copy, genuine Eagle ones being prohibitively expensive (and sometimes difficult to obtain in proper Coyote Brown, since you want to do MFR). Couldn't say how other copies fare, depends on how fussy you are etc. and you'd have to check reviews.
As for the PANTAC RRV, I have mixed feelings about it; it's superbly built, although there are a few minor differences with the real Eagle RRV (namely some of the Fastex buckles and the D-rings for the bib, otherwise it's almost dead on). My biggest gripe with the PANTAC RRV is the shoulder straps (padding) are much stiffer than Eagle's and I've found it extremely uncomfortable or sometimes almost impossible to wear with the shoulders strapped across the back. The genuine Eagle has much more supple shoulder straps and doesn't pose this problem.
With the rear panel installed, however, the PANTAC RRV is excellent.
The genuine Eagle RRV is a bit pricey but still affordable for airsoft if you're willing to invest a bit in gear.
Other brands, YMMV. Bean has a review of the FLYYE vs. PANTAC RRV. Anything else will probably be below PANTAC. Again, depends how fussy/cheap you are.
Of note, the PANTAC MAP has a few minor design differences compared to Eagle's, but I find it superior overall because of them (build quality is identical).
Pic of PANTAC RRV with a [wrong color/khaki] rear panel from a plate carrier. I was using the KA PACA under it.
http://tacticalmoose.com/mirabel-06-06-09/060609_15.jpg