Newer units had USB transfer disabled, which might be what they were talking about, but there is a custom kernel available for all currently known new units that enables USB, and adds features such as brightness control working anywhere and the ability to shutdown the device from anywhere by holding down the power button for 10 seconds. This makes new units as good as or better than old units, with the exception that new units can?t use reGBA, the better of the two GBA emulators for the RS97.
I?ve heard of a couple instances of the screen being too bright or some units not supporting overclocking as well, but it sounds like most people are happy with their units. Plus you could always get a refund if you get a bad unit. I?d say either go for it now or wait for the plus. (The pro has the same CPU, so don?t expect better performance or anything.)