The Tekno is made to use the clutchbell so it positions the motor further back. You can use a pinion, but you either have to use a long shank type to make it reach (and may place more lateral load on the motor bearings), or modify the mount so it sits closer to the spur.
The RCM mount is designed primarily for the typical pinion setup. Simple, yet effective. However, you cannot use a clutchbell adaptor because it does not have the clearance the Tekno kit does.
Which one should you get? Depends. If you even think you might want to use the clutchbell adaptor, get the Tekno. Then, if you decide not to use it later, you can mod it as stated above to use a regular pinion, even though it may be a bit more cumbersome as it is not designed to be used this way. But, if you know you will only use a regular pinion, get the RCM mount.