I had similar decision to make. I have the same camera and housing and wanted to order a strobe to go with it. I looked at both the YS-110 and the YS-27. The specs of both strobes are almost the same except that the YS-110 is a TTL-strobe and costs more. I bought the YS-110 because of the TTL-option. It gives you a bit more flexibility although you still have to select the aperture depending on the distance (range), but this is less critical than with an non-TTL-strobe.
I am still experimenting to get the best light for the picture. I am not completely happy with the results up till now. It seems as if the pictures are consistently underexposed which should not be the case with a TTL-strobe unless I grossly underestimated the distance and chose the wrong aperture. I never had this problem with one of my previous cameras, the Nikonos 5 with the Nikon SB-105 TTL-strobe, so it must be something with the camera and/or strobe. It is easily corrected in Photoshop when working in RAW-mode, but the exposure should be better from the start. Macro-shots are always good provided the strobe is aimed correctly, so it seems it has something to do with the distance and guidenumber. Maybe the guidenumber is overstated. The Nikon SB-105 has the same guidenumber, but provided better exposures. For the time being I blame it on me and not on the strobe.
So if you have the money to pay for the TTL-strobe I would advise you to choose the YS-110; otherwise you should choose the YS-27 and more carefully select the proper aperture.
One word of warning though. You have to work with the camera in manual mode. Automatic mode doesn't give good results because the software doesn't seem to be correct for flash-exposures. In automatic mode the camera still tries to get a good exposure without flash which results in long exposures with no control over the aperture. In manual mode the camera uses the LCD-brightness to show you if you are over- or underexposed. Under water this will always result in a rather dark LCD-panel wich makes it almost impossible to evaluate your focus. This is a major flaw with this camera that hopefully will be corrected through a firmware upgrade in the future.
This afternoon I go diving with a newly bought set of rechargeable batteries to find out if that solves my problem with the strobe. I will keep you informed if this improves the exposures.
You can check pictures taken with this camera at the following links:
Picasa Web Albums - John; all pictures are taken with this camera and strobe. The most recent are the pictures of the Superior Producer wreck. The first picture of the octopus show the (corrected) underexposure the best. It is grainy because I had to correct the exposure so much. As you can see, most pictures are good, so my problem is partly that I am very critical on myself.
Welcome to the Curacao Diving Paradise - Curacao Diving Paradise. This site contains pictures of all three my cameras. When going through the links on the home page you get recent additions to the site and these are all taken with the Sea & Sea DX1G and YS110.
Hope this helps.
Gustele