I checked them both out, side-by-side when they were released. 'Depth' seems to often be an over-hyped topic, at least to me. because with over 56 years of avid detecting I have noted that most old coins, trade tokens, gold and silver jewelry, and all sorts of desirables are nt all that deep. Oh, I want 'ample depth', but I generally get that from a good quality mid-size coils that is a round 7" or 8" diameter, or elliptically-shaped of maybe 5X8 or 5X9½ size.
In my comparison tests, there was an indication the round 8" might provide slightly better depth. However, the 8" was also noticeable heavier than th 5X9½, and it also balanced worse because the rod-mount 'ears' are too far to the rear of the coil rather than more centered. Therefore, the 5X9½ resulted in a better balanced and lighter-weight coil which made it a lot more comfortable to put in a longer duration of hunt-time.
I keep the 5X9½ DD mounted full-time on my Simplex +, and honestly feel like SvenS that this model should have been released with the 5X9½ DD as 'standard' and not the bigger round 11" DD. I also think it should have had a round 5" DD offered for it as well, and even their round 7" Concentric coil.
Monte