No experience with the DATA, but the way I see it, there are different focuses.

The O'Tool is compact and powerful - two traces, extensive (and accurate) tuning and voltage measurement in 8hp.

The Data has 4 channels, and some extra clock and oscillator functions.

Can't recommend the O'tool + enough though. Tuning has been great, its just the right size to visualize information without taking up too much valuable HPs...