Fmaderjr said pretty much what my response was gonna be. Remember that the recorder part of a digital recorder makes no difference...it's simply writing 1's and 0's. Where there could be a difference is upstream of the recording function...in the internal preamp and ADC of the machine. On the other hand, as already mentioned, for these devices there tend to be differences when you use electronic gear to analyze down to the gnats ass, but sonically I'm not so sure you'd hear much of a difference between the two recorders you're considering. The predominant influence on that aspect of your recording will be the stuff that you've already purchased.
EDIT: Sorry to be blunt, but the previous post is just a little self serving, without providing any reasons or justification why the iriver is a good alternative for this noob. I'd pass on the iriver personally. I've heard they're nice machines, but 16bit is old technology and since you're just starting, start with 24bit. 24bit provides better resolution, which in itself isn't all about better sound as much as greater dynamic range and most importantly greater headroom so that you don't have to run really hot to get the best sounding recording. Running with alot of headroom enables you to avoid clipping during the show because you can set the levels with lots of spare headroom without having to worry about checking levels every 5 minutes. So, on top of everything else, running with added headroom (say between -10db and -5db) has an intrinsic benefit of enabling you to enjoy the show more because recording becomes more of a 'set it and forget it' exercise in 24bit.