The 55 is a great airplane for the money. I just spent 3 hours in one today tooling around Texas. It is a dog compared to the rest of the Lear fleet. There is no comparision on the 60. A 60 will outrun ATC's computer that displays your altiude on the controller's data block. Rocket ship.
The 55 is plenty fast in cruise it just takes a bit longer to get up there. The biggest limiting factor is what type of missions/loads you intend to fly. As previously stated, the 55 is a runway hog. If you plan to routinely frequent rural strips, and then depart and fly halfway across the country, then it's not your plane. If you are planning on routinely visiting ASE, TEX, EGE, GUC, then forget it, unless your owners don't mind stopping immediately for fuel.
For instance, ASE on an 80 degree day at 16K and 4 pax=2500 pounds of fuel max. (Just gets you to Pueblo)
Or;
Long trip with a full airplane (Fuel & PAX) from sea level on a 100 degree day: 7810' field length. O.K for a nice big airport, but it does limit the utility of the jet.
The 55 is nice to fly and PAX like it. (First jet I took across the pond.) Lot of airplane for the money. PM ime if you need any help.
Good Luck!