Looking for recommendations on either O&M or Selectapension (ideally) please, or possibly Synaptic Comparator.
Neither. Save your money, use a spreadsheet
Seriously though, if you do a search on these names in the site search bar there are several existing threads with various aspects being discussed.
Having done a fair bit of research on these recently, we're seriously considering Adviser Asset's offering. It doesn't try to be anything other than a switching tool, and is priced as such. Lovely and clean to look at without all the additional bells and whistles that we would ultimately pay for and never use with the others.
Used to use O&M and their new tool does look very flashy; however we have used Adviser Asset for a number of years and find it very good for our requirements with ability to add special deals/DFM costs etc.