I have three 20 lb. tanks here and two 5 lb. tanks. I use the 20's for things like pressurizing the fermenter after dry hopping, purging kegs, and for close transfers. One is also used to serve the beer out of my keezer. They usually last a couple months before I have to refill them which is why I keep spares one around. The five pounders are only used to carbonate the beer in my lagering freezer because of space considerations.
IMHO, I would go with the 20 lb tanks for closed transfers so that you don't have to refill them as often.