I see you made the leap. I think I would go with the Tascam too.
First the Tascam has the line/inst toggle
Someone mentioned the Steinberg having option for second USB power,,, I do not see that looking at the pictures, so the Tascam wins again having the A/C power option. And the Tascam "looks" solid and has all the good stuff on the front panel.
Steinberg is Yamaha and they certainly are a company I trust.
We bought a Yamaha mixer with USB for the band and it uses Steinberg ASIO drivers. Very reliable and had no problem co existing with my Tascam drivers. I could put my Laptop in WDM mode and use both side by side. So both companies are writing good drivers.
Pre amp wise both are probably in the same camp. I have a small Yamaha mixer which probably uses the same off the self pre's. Every manufacture will claim that their pre amps are awesome. But you get what you pay for with Pre amps and A/D convertors these day's.
My favourite test is using headphones, turn up a pre amp and see how noisy it gets with nothing plugged in. That's why I bough my Yamaha mixer ,and a Mackie. The B brand was the worst and A&H close behind.
My Tascam interface is also pretty quiet under this test and I read they have uped the quality on the new models,, this is why my next interface will be the Tascam UH 7000.