The GIV-SP (GIV) shares a type with the GIV, the G400, and the G300. EASA only requires familiarization training between all.
The Challenger 604 (CL604/605) shares a type with the Challenger 605 & Challenger 650. Differences training is required between 604 and the 605/650.
The GIV-SP, GIV, G300, and G400 have 7, 4, 1, and 1 models in Europe respectively as of DEC 31, 2018. (13 Total)
The Challenger 604, Challenger 605, and Challenger 650 have 55, 45, and 16 models in Europe respectively as of DEC 31, 2018. (116 Total)
If type-ready crews are a priority, Challenger could be a better bet.
As some in this thread have said, the right people are always a good investment.
Hope this helps.