Vega owner here. I'm toying with the idea of trying the Taurus-pre and also using it's headphone out. My listening has tilted more towards my stereo rig lately. The Taurus pre supposedly has a more illuminated top-end compared to the headphone-specific Taurus. I don't suppose anyone here has had a chance to compare the two. I'm a big fan of my Vega and am about to digitize my vinyl to DSD.
That's what I hear as well, regarding the lit up top end. Apparently the sound is intended to work better with the frequency response and dispersion characteristics of speakers as opposed to headphones. The Taurus mkII headphone amp can also work as a (simple) preamp, and the Taurus Pre does have headphone amp capabilities, but each is supposedly better suited for its primary function.
Personally, I'd follow their recommendations. If you mainly want it for headphones, get the mkII because it is meant for that and has balanced as well as SE outputs. It does well as a preamp but with only a few inputs/outputs it's not really meant to replace a dedicated unit. If I needed a preamp, I might go for the Taurus Pre, and get the added bonus of a good headphone section as well... that assumes speakers take priority over headphones in my setup.