Lifetime service stays with the machine.
All other service contracts stay with the account. You can freely transfer service between any non-lifetime machine on your account, but may not transfer ownership of that contract.
If you only have one machine on your account, you can work around it, but that would be a violation of the TOS.
If you think you are going to have Tivo for more than a year, I'd go with the 3 year MSD.
If you decide to sell the THD before the contract is over, get a used box and transfer the service to it as a "holding place" for the remainder of the service.
All other service contracts stay with the account. You can freely transfer service between any non-lifetime machine on your account, but may not transfer ownership of that contract.
If you only have one machine on your account, you can work around it, but that would be a violation of the TOS.
If you think you are going to have Tivo for more than a year, I'd go with the 3 year MSD.
If you decide to sell the THD before the contract is over, get a used box and transfer the service to it as a "holding place" for the remainder of the service.