You only appear to be able to buy the one-time updates from Garmin, haven't seen them sold elsewhere. On the other hand the life-time updates are sold by the like of Handtec and Pentagon GPS.
Look at the prices above and make your own mind up...
I believe that the updates via Garmin can be downloaded straight away using your myGarmin account whereas the updates purchased from other vendors require that they send you a card with a code that you use to obtain the mapping, again via your myGarmin account. If you're off on tour in three days time then a/ buy the update from Garmin, and b/ your froward planning is crap
I was half joking about the forward planning thing, wasn't aimed at you but just illustrating the only scenario where you might want to buy the one-time update against the life-time one.
Having said that I bought a one-time update a couple of years ago for my SP2720 and found out subsequently that I should have bitten the bullet and got the life-time one, not sure if you could buy the gift card versions then though.
More recently have bought a life-time update for my Nuvi (that I now use on the bike) from Petagon GPS. It's easy to register and get set-up, there's a link within your myGarmin account called 'Activate Gift Card'.