two cents worth - I was disappointed with the audio quality of the one Sapphire that I tried. Wasn't my studio, so that could have been a factor. Several folks that I respect say that the new Tascam is awesome. If I had not already pulled the trigger on the Presonus 1818VSL I'd probably be torn.
As it is, I am quite happy with the 1818VSL. My reasons for selecting it are not terribly scientific, but if you are curious - first, I know one of the engineers that worked on the 1818VSL, I worked with him at M-Audio and I trust him to do a great job. Second, I was looking for almost the exact feature set - 8x8 analog audio, 1 or more lightpipe, 1 or more S/P-DIF, and at least 1x1 MIDI. At the time I did not expect to need the front panel controls and headphone jack, but I may end up mounting this nearby.
My first choice was the UAD Apollo Quad, but that was out of reach this time around. When I get it I will keep the 1818VSL as my mobile interface... unless something better comes along<G>!
So set your budget first, set your requirements second, if they don't match start again. Audio quality is so good on all but the bottom of the barrel that it comes down to degrees of difference, or maybe even just personal taste. So if you can, listen to the candidates. And of course ask here and elsewhere about drivers and support.
Hard to get the best, almost as hard to go wrong... so my goal was to avoid going wrong, and stop worrying about the best... at least until the meds wear off!