I suspect it will come down to the type of work you are doing. A single colour GTO sure can do a four colour but not efficiently/competitively so it's a bit of a moot point.
I'm not sure, but I'd say the PM would make ready a two colour job pretty quickly so if that's your market then it might be a good option.
Are they a portrait press? This may reduce thier capacity to run a decent solid. Roller power is important.
Are they reliable? GTOs are bullet proof!
If you had the floor space I would say put the new PM next to the GTO. You can make more money in a week with a GTO than it costs to buy one (or what you can sell it for)..........