I can't answer to whether the retired police dog or a puppy would be best for you. That said I think the more important question you should be asking yourself is how either would impact your senior dog. He has been your best friend and protector for 13 years (assuming you got him as a pup). He may get along with other dogs on the short term visits but bringing another dog into the home is a whole different situation.
you could suddenly see jealousy and aggressiveness from you senior dog. Or you could see depression and withdrawal. Old dogs are like old people, they can get very cranky with big changes.
Many here argue that to many of us humanize our dogs and say we shouldn't blah, blah, blah. But lets face it, after 13 years your senior dog has a deep bond with you and may not take kindly to you replacing him(in his eyes) or tormenting with have to deal with another dog in the house. Or it may all turn out fine and he could be ok with another dog, adult or puppy. But it is something else I think you should consider before you get either.
Best of luck to you.
Ziva GSD 3/7/13
At the Bridge: