You don't say why you are thinking of moving off SD 4.5. Without that it is hard to give recommendations.
Unless there is functionality that 4.5 cannot deliver that you must have then I would stick with 4.5 but go to the latest service pack.
For new implementations the recommended product is Service Center because of its increased capability - but it takes more time to implement compared to 4.5. There is no upgrade path from 4.5 to Service Center (but there will be to Service Manager).
There is an upgrade from 4.5 to 5x but it is not very reliable and 5.x as a product has not been well received. It does have some nice new functionality (map views, mandatory fields based on complex criterion, multiple locations etc) but is also missing some of the features of the latest service Packs of 4.5.
I would say stay with 4.5 and apply the latest Service Pack. (personal opinion)
I would agree with Jonathon in this case. 5.x is not worth the trouble and the migration tools are still not available to public (AFAIK). AFAIK there is not migration tools at all for Service Center so you would have to do everything manually. Stay with 4.5 and apply the latest SPs.