Dealer misrepresented a base model as a touring

I'd say the choices are different for 16" rims but not more restrictive. Some higher end tires aren't available in 15" such as the Michelin Primacy MXM4 and others.

Now if your Prius has 17" rims you definitely have fewer choices and an even bigger loss in fuel economy (although still only a few percent).

The nice thing is you can buy a Touring Prius and put 15" rims on it. Then you get the better suspension and the better fuel economy/cheaper tires.

If I had 16" rims I wouldn't bother switching back to 15" unless I was doing summer/winter rotations with two sets of tires.

If I had 17" rims I'd be happy with either 15" or 16" rims for downsizing.

recap on OEM rim sizes

15" Rims (standard aluminum rims in the US 2004-2009)
15" Rims (standard alloy rims in the US 2010+)

16" Rims (Touring Package only in US 2007-2009 magnesium rims)
16" Rims were standard in other countries but I don't have the specifics

17" Rims (2010 model 1229 aka "package V" alloy rims, 2011 "Plus Performance Package" alloy rims)

So if you see a 2010 or newer car labeled as Touring it's misnamed but it might have 17" rims and other features you might think of as "touring" like.

And if you see a 2009 or older car with 17" rims it doesn't mean it has the "touring" package. For that matter an unscrupulous seller could put 16" rims on a non touring Prius to make it harder for you to notice that you aren't getting what you are paying for.