It seems to me that you are to a certain extent comparing apples to oranges.
An Omega 4x5 enlarger in reasonable (i.e. undamaged) condition is capable of very high quality results. They are also robust, easily maintained, and convenient to use.
By all reports, the Devere equipment is top of the line, so I would expect similar if not better results.
If you were asking about which enlarger to buy and set up yourself, my sense is that most in your part of the world would say as long as they are in reasonable shape, the Devere is preferable.
The difference, however, is that you are comparing an enlarger that is not set up, which you intend to use in a room not configured for darkroom use, vs. an enlarger that is set up in an operating darkroom.
I would base my decision on whether I could get the Omega cleaned, aligned, functioning properly and equipped with the proper lens, negative carrier and (possibly) condensor set. I would then ask the same questions about the Devere. If there is a real difference in time lines, you probably want to go for the quickest option.
Even if you decide to work with the Omega for this project, it sounds like it would be an excellent idea to retain the Devere for the future, if it is in good shape.
PS Ilford has manuals on their website for some of their multigrade heads. Harry Taylor's classic enlargers site and KHB photografix's site both have oodles of info about Omega enlargers, and KHB also has a Devere Enlarger resource too.