As far as I know the specific differences between the various VSTi content sets isn’t documented (at least not publicly). But you can see in the chart that going from Artist to Elements more than doubles the available instrument (>1000 to >2600). I only have PRO so can’t tell you if the specific piano sound you liked is included in Elements or Artist - perhaps someone who has those will comment.
I did find this thread of the differences in Halion you might find useful.
However I wouldn’t let the included sounds drive your decision too much. This is because there are tons of good quality third party instrument sounds readily available - ranging in price from free to modestly priced to arm-and-a-leg. KVR Audio has a database of VSTi/sounds that is a good place to find free/cheap plugs.
You can see from the Effects section of the chart that most of the amp & distortion plugs are in all versions, so that should be covered. And again you can find additional effects at KVR.
Also Steinberg usually (no guarantee) has an upgrade sale late-summer/early fall. So you could get Elements now, use if for awhile to see what you find lacking in it and then upgrade if you want to a bigger version with the sale.