The Déus & ORX can do quite well with a round 9" DD. I've had that coil, but I prefer the elliptical DD that's a 5X9½ (CM to Inches would be 5.12X9.45 so I'll just round that off to 5X9½). It's what I consider a 'mid-size' coil while a round 9" or 11" falls into my larger-coil category.
On the Simplex + I would have liked a round 7" Concentric coil, like they make for other models, but sadly they limited their coil offerings for the Simplex +. I got to try both the round 8" DD and elliptical 5X9½ DD and, by far, the 5X9½ was more comfortable. and just a slight loss of depth, but overall impressive performance. The 5X9½ is what I ordered and use exclusively on my Simplex +. The rod-mount point is also better on the 5X9½ as it is closer to center. I didn't like the 8" because it was quite heavy, compared to their other coil fferings, and has a rod-mount that is too far toward the rear of the coil and that makes it balance poorly and it gets easily bumped / knocked out-of-alignment to maintain a flat, horizontal orientation to the ground.
I spend most of my hunt time in very debris-filled sites so th closer masking trash hampers trying to achieve any 'extra' depth, and most coins, tokens and desired targets are in the surface to 4" range with 5" to 8" or slightly deeper being achievable a smaller percentage of the time, so I am not a big 'depth' demander out of a detector and coil. I had 3 ORX devices w/9" and 5X9½ and got excellent performance out of the elliptical coil for desired target responses on 'deeper' keepers. I also enjoyed very functional deeper-coin performance with Vanquish 540 w/5X8 DD, my Apex w/5X8 DD, and a 5X9½ on the Racer 2, Impact, Multi-Kruzer, Anfibio Multi as well as my FORS Relic and Simplex +
I won't 'ridicule' you, but I preferred th 'S' rod package with the Racer 2 over the heavier-feeling Multi-Kruzer. I used the Racer 2 a lot, and have used many 'S' rod detectors through the years, along with my current two favorite Tesoro model and three Apex devices with different search coils. I like the Racer 2 w/5" DD and 7" round Concentric, but not with any of the other-size coils. The Multi-Kruzer, when tried side-by-side with a Racer 2 and same-size coil, felt heavier and 'less friendly' to me and two hunting buddies when we did some comparisons.
I prefer my Simplex + w/5X9½ to the Racer 2 w/5X9½ and let my last one go. My balance and mobility are very impaired and I've been using a cane since March of '93. I do my best to not rely on a detector to serve as a cane / support .... but have on a few occasions just a little. Some makes & models felt / feel more rugged or durable and can be a little more stabilizing at times .... if absolutely necessary.
On a new SMF I would be fine with a round 11" DD as standard, but want a 5" and 5X9½ DD plus a round 7" Concentric to be available as accessory coils.
Monte