Out of these two cartridges which would you choose?

Depending on the output sensitivity of your amplifier, the Dyna’s output coupled with the SLP98’s 43db gain may not be enough to drive your amp to full power. The 2M Black may be a better match

For MM, perhaps one of the Nagaoka's may also be a nice choice. Assuming a HO will work, you might consider one of the Hana HO carts.
But I suggest calling a Rega dealer who carries a few of these, because the arm and cartridge synergy is so important. I only have direct experience with the 20X-L, and it's a very even handed cartridge that does nothing wrong. You could easily do a lot worse. No direct experience with the 20X-H though.

I have a ortofon 2 m bronze on it right now and only need the gain nobs half way turned up. So the preamp should have more then enough gain for 2.8mv, plus I have it hooked to a cary cad120s power amp which is an awesome match. I have always owned moving Magnet but never a moving coil. From what my rega dealer told me, In that case I should try the dynavector 20x2. And go for the high output version then I don't need to put more money into a new transformer. The only thing about the 20x2H is its exactly very much the same as the low output version. Though theres a little more zip on the higher top end. But that should be buffered out because of the tubes. And hearing a lot of success with rega and dynavector in the past.

The low output Dyna 20xl is a much better sounding cartridge than the HO version. Trying to get more output from a MC cartridge is a design compromise which "typically" translates into lesser performance. I am not saying the HO Dyna is a poor sounding cartridge, but it is not the same league as the LO version. BTW, the 2M Bronze and Black have almost twice the output when compared to the Dyna, therefore, as the previous poster mentioned, make sure the Dyna is an electrical match.

I had a Dyna 20X-H on a Rega P7 (RB700 arm?) table and the combination was a great match.  My then dealer suggested the cartridge when I asked his opinion.