Some will say learning to use the #45 is a prerequisite to using the #55. There is one #55 in my shop and four #45s that get used.
For most things one of the #45s is used. Recently a #50 was purchased. It is set up to do the 1/4" plow work like box and drawer bottoms.
For all but the complex shapes the #55 stays on the shelf. It is a bit fussier than the #45 to get working.
My advice is to purchase the #45 being offered if it is at a good price.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)