What varies greatly with bike sizes is the length of the top tube and so the distance between the seat post and the handlebar stem. The stem can be replaced to increase the distance from the seat to the handlebars by an inch or two but the frame size still should be in the ball park for the rider. I have 4 bikes, both road and mountain bikes, and the distance from the seat to the handlebar is the same on all of them.
When I am adjusting the seat on a bike I ride for 15-20 minutes to warm up my legs before I make any adjustments. Otherwise what feels comfortable with my muscles and tendons not being limber is not going to provide an accurate feel for prolonged rides on the bike.
It is also very different to be doing riding in town on city streets where a more upright position can be helpful and where wider tires to absorb road bumps makes for a more comfortable ride. Different on gravel and dirt roads and still different for riding on hilly terrain on paved roads. Where one expects to be doing most of their riding affects what bike is better suited for them.
There is no substitute for experience and some value if one is starting out to buy a used bike and spend some time on it before spending a lot of money on a new bike. There are also some excellent bikes for sale on Craigslist and one can save more than 50% of the price for these bikes when new.