Being the proud owner of both a Scarlett 6i6 and a Tascam us1641 I will say with full authority, Don't buy the Tascam if you fear bad drivers. 
The us1641 is the older model, but I would hazard a guess the drivers share 98% of the code. The us1800 was more or less a face lift to the front panel. I would say go and buy it if all your going to use it for is tracking live drums. It's certainly the most usable inputs for your money. 
Tascam works real good for audio tracking, no problems, but it just would slowly get worse when you started adding soft synths and effects to a project. And it's not about latency, I'd say both the Tascam and the Scarlett are close with RTL.  Both are spot on for overdub timing. Both trigger MIDI with just a hare of delay. But there's more than that to audio drivers and how stable they are with Sonar. Focusrite Drivers likes Sonar as many will tell you. 
 
When you talk low latency don't be fooled by direct monitoring babble about zero latency. Of course it's zero because there's actually no latency involved when you monitor directly. There is no difference in monitoring latency between the Tascam and the Scarlett or any other interface for that matter. It's Round Trip Latency performance that separates the men from the boys with interfaces. 
It's when you monitor the output of the DAW while tracking that RTL will bother you. 
 
In your case, if you wish to track your USB drums while playing a VST drummer you'll need better performance. I have to monitor using my Yamaha DTX Drum Brain because it's much tighter timing for me. If I trigger a VST drum it's a 128th note but it bugs me. But the brain is using it's own USB drivers and so this might be a Yamaha driver issue, not the audio drivers. 
 
 Read this thread if you need to get a handle on what is good and what is bad about interface RTL, I bookmarked page 13 as that's more up to date charts. 
 
http://forum.dawbench.com/showthread.php?1548-Audio-Interface-Low-Latency-Performance-Data-Base/page13
 
 
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