My feeling about nozzles is that softness is unimportant in achieving insertion if the nozzle's insertable length is less than the distance to the first bend in your sigmoid colon (12 inches for me), and it's less than 75% of the diameter of the largest butt plug that you comfortably enjoy. I've found that whatever diameter I can get through my anal sphincter, will always go through my sigmoid (or upper) sphincter too if I get the direction right, (toward the back aimed at the tailbone), and I do a little warming up first and some playful prodding. Penetrating the sigmoid sphincter is EXTREMELY pleasurable. I think you would feel the pleasure more with 18 Mildly Firm.
I do agree arthur that soft nozzles can be harder to insert because they are more flexible and can double back. I don't think 10 Soft would have this problem, but 00-50 Plus Soft might not be firm enough to easily push the Model B through the sigmoid sphincter for some people. I've got a 10 Soft, five-foot colon tube that is easy to insert most of the time. I think every nozzle or tube has an optimal softness, so when in doubt use the middle sofness grades available.
For the Model B, the 1 7/8 diameter of the sealing egg for the sigmoid sphincter should not be a concern if you can easily handle a 2.25" butt plug, and I'd recommend 18 Mildly Firm. If on the other hand, 1 7/8 inches is a huge plug diameter for you, go with 10 Soft or 00-50 Plus Soft to make insertion easier.
The other concern is your distance to the first bend in the sigmoid colon. For me it's about 12 inches, so the Model B's length is not a concern. This distance varies greatly from person to person. If you know yours is under 10 inches, go with 10 Soft. The 7/8" guide tip will be more than flexible enough to make the turn in 10 soft. If your Model B has to make that turn, it might not bend enough to do it in 18 Mildly Firm.
Here are the nozzles and I have and my experience with them. I can barely handle a 3 inch firm butt plug, and the distance to the first bend in my sigmoid colon is 12 inches.
DBLBL-3XL in 18 Mildly Firm, (2.5 inch head and 10" insertable length.) This nozzle is very similar to the Model B. It's insertable length is about the same. The diameter of the head that penetrates the sigmoid sphincter is 2.5 inches verses 1.875 for the Model B. I LOVE my DBLBL-3XL in 18 Mildly Firm. I take my first enema with its sigmoid head in my rectum, and then push it in all the way for my second enema.
SPD3.0 - 3 Inch Diameter Super Retention Silicone Nozzle (3.0 inch diameter and 6" insertable length) in 00-20 Extra Soft
SNCMAX3 3 Inch Diameter Maxi Retention Nozzle (1.8-2.5" colon diameter, 10.5 inch insertable length) in 00-20 Extra Soft
These two retention nozzles are 3-inch diameter, right at the top of my butt plug tolerance. In In 00-20 Extra Soft, these babies both pop right in, seal perfectly, and feel great. They are so comfortable that I leave them inserted for very long periods--as long as two days! I use a plastic toilet bolt to plug the bore hole so they don't leak when I'm out in public. The only downside with the supper soft material is that it takes a high diameter connector adapter to keep the enema tube from popping out. Definitely get the small bore size.
CT-152French 2 Inch Diameter Silicone Serpentine Colon Tube in 12 Semi Soft (20-inch insertable length) You could consider this an extra-long nozzle. It goes six inches past the big turn in my lower sigmoid. A lot of times I have trouble getting it to make that turn. I should have gotten it in a softer material. I failed to take into account that larger diameters of he same material are less flexible. I don't use it much. I might try to whittle down the tip to make it more flexible. I really want something this diameter and length that I can always get in quickly. I'd use it to straighten our my sigmoid colon to let me use my 5-foot colon tubes on days when they are getting stuck in my sigmoid.