I depends on the site conditions and what your willing to tolerate. A quick point, a (5) semi-auto setting can totally blow away a (5) manual setting in terms of depth if the conditions are right. Semi-Auto should be thought of as Default, and manual should be used when and if you are able to judge the ground conditions.
I've used different sensitivity settings to fit different sites but more importantly I've set it more times to the type of hunting I want to do.
Many don't realize that at full(32) Manual sens that you can hit a coin as deep as any other setting you can try and possibly deeper. I know this because even though I was more interested in continuing with what works I also wanted to test it out. Well, it works great, I challenge anyone to find a Deepie in there normal set up then go to 32 manual and check it.
But careful now, 32 manual isn't for every one and it must be used at the right place and the right time, in other words you have to hunt smart. Don't go in a high trash site to test this out, hit an open field first, just like everything else you have to learn what to listen for. It's advanced hunting and when you get some deepies using it you won't believe it.
Anyway, the site dictates the sens setting, sure set it at 18 semi-auto and like the commercial says 'set it and forget it,' but that's the beginning of getting stuck using one setting. You have to go into a site and start detecting, if it's trashy you go either low Manual or a little higher Semi-Auto, if the site is clean then you go to a higher manual setting say 22 to 26, or 28 to 32 semi-auto. If the site is super clean it may still have high mineralization so clean doesn't mean everything so if the ground has higher mineralized soil then a much lower manual setting say 14 to 18 may be necessary or a Semi-auto setting of 20.
Either way, I would say to try and figure it out this way. This is a GENERAL GAUGE:
Super clean, low mineralization- (32) Semi-auto; (31) to (32) Manual.
Clean but some mineralization- (2

to (32)Semi-auto; (27) to (30) manual.
Trashy and high mineralization- (20) to (30) Semi-auto; (1

to (24) manual.
super trashy hot ground- (10) to (23) Semi-auto; (6) to (16) manual.
The trashiest and the hottest ground- (10) or below Semi-auto; (

or below Manual.
This gauge is only from the type of sites I have hunted and are going to be different from others so again it is only an example of how you can judge where to set your sensitivity. My point earlier about (32) manual is just to state that it is a viable setting option and very useful when you have exhausted all other settings and are ready to use the raw power of the XS, and yes to much sensitivity can be bad but again ya gotta try it some time if you want to get a shock.
Semi-Auto is great, but so is manual. You have to decide which to use when you get to the site.
I think of Jim (UpstateNY), at the GNRS hunt, how well would he have done if he refused to make changes because he was stuck thinking that the settings he used at home in NY were the best and only settings to be used at the Virginia site where they held the hunt. If I remember right he adjusted to Semi-auto and began hitting many more targets.
I have gone to new sites many times with an entire set up already planned and thinking the how great these settings were going to work but when getting to the site ending up throwing it all out and using a totally different set up.
Just my thoughts on this, I hope it's some help.
GL/HH