Be very carefull what you get, there has been a bit of talk recently about the speed rating of " Trials" tyres used on Dirttrack.
There's been several examples recently of tyres exploding at high speed, thankfully without any real serious injury but I wouldn't like to be on one when it went off.
There is also talk of differing quality of tyres within the same model and brand, eg - country of manufacture.
If I were you I'd be going for a Mitas, only because I haven't used anything else in recent years.