Lyr has way more power at low impedance; Valhalla is designed to deliver at high impedance. From my experience with the T1 at 600ohms, even with its Tesla efficiency some amps can feel that it's a stiff load and lack dynamics compared to a more muscular amp. Even my Meier Cantate that does well driving the AKG K70x to have dynamics comparable to the HD600 didn't do too well on the T1, which sounds a little sleepy compared to the Sennheiser and AKG (and even the DT880).
If you can afford it, go for the Lyr, even with 100mW less output at 300ohm and 600ohm. In case you decide to get orthodynamic headphones later on the Lyr will be ready for 'em too, more than the Valhalla. Of course, if you're somewhat sure you'll stick with higher impedance headphones and won't get into very demanding low impedance headphones (the Valhalla2 supposedly does muchbetter on low impedance loads than the original version), then the Valhalla2 is a good choice.
Tradeoffs. Your the input impedance of the T1 is 600 Ohm, so Valhalla will actually deliver more power. Also, Valhalla is all-tube if you like tubey sounds, while Lyr is a hyrbid (so it may not be as tubey sounding).
But that hybrid design means that Lyr can offer much more power into low impedance, while Valhalla falls short. This puts Lyr into a "can power pretty much anything" category that's great if you're hopping around many different headphones.