I don't have any hands on experience with the Taig mill, but I do have a Taig lathe and a Sherline lathe and mill.
Speaking in general terms based mostly on the lathes:
Taig seems to excel in making things inexpensively but good. Not in a cutting corners way, more just not making something cost more than it needs to and with the idea that the end user can always upgrade to add features if desired, they do offer many of these upgrades just extra to the basic machine.
Sherline is more refined, better finished, more features on the basic machine but also significantly more expensive. Base model lathe only package for Sherline is $625, same for Taig is about $400 (adding in the cost of a tail stock which is not included on the basic Taig packages).
Both are highly customizable but Taig seems even better suited for those who want to make a custom machine. You can literally buy a Taig one piece at a time, and they are set up for this so it does not become ridiculously expensive. You can also buy most any Sherline part, but it is far more affordable to buy it complete.
Taig is a bit larger, the base Sherline lathe is a 3.5x8" lathe, Taig is a 4.5x9.75" lathe. The Taig mill is substantially larger / heavier at 80lbs vs 30lbs for the base Sherline 5000 mill (Sherline has 3 distinct mill families 5000 series, 2000 series and the 5800 series).
Sherline offers manual, and CNC ready machines up to complete turn key CNC packages including all the hardware, software and even the computer. Taig is a little more build your own for CNC, they offer packages that get you about 80% of the way, but the last bit is on you.
My preference is towards Sherline, but I find Taig has many clever solutions to how they do things. If you want to tinker and make your mill exactly the way you want it Taig has a definite advantage. If you just want to buy it and get to making stuff, Sherline will get you there faster. My experience is that both companies offer very good customer support.
Taig has a pretty dreadful website though, very 1990s Geocites-esqe. They do have a reseller Nick Carter who has a great website, and is a big advocate for Taig. I'd dig through his site and read some of the articles he has posted, maybe even send him an email if you have specific questions. He is active on a Taig forum and seems to be very open to talking to people about Taig machines.
http://www.cartertools.com/