Round shaped toilet bowls have been on the market practically since toilets were invented. Elongated toilet bowls on the other hand are a more recent design. Why this design appear and become popular?  Below is the detailed information:
Advantages:
Space-saving
This type of toilet bowl will usually measure anything from 25 to 28 inches from the wall to the front which means if you have a small bathroom, this type will help you save space, and your door won’t be bashing into it every-time you need the bathroom!
More affordable
You can expect to pay a lot less for a toilet with a round bowl simply because the design has been around for so long, and manufacturers need to use less technology in order to produce them.
More comfortable for child
If you’re potty training a child, a round bowl is the answer. Any parent; will know what a messy time this can be for you and your child so if you’re just making the transition from potty to toilet, a round bowl model could be the answer. Round bowl may be more comfortable than an elongated because you don’t need the extra space.

Disadvantages:
If you’re a large person, this type of toilet will probably not be too comfortable to sit on.
Some models may not have as powerful a flush as the elongated type because there is less space to play with in terms of technology.
 
Elongated Toilet Bowls
Advantages:
More comfortable for adult
Because of the shape, this toilet design is much more comfortable for adults as a whole as well as for people who are on the larger/taller size.
More Powerful flush
Models like this tend to be on the powerful side when it comes to the flush. There are two reasons for this, the larger water surface area and the space available so up to the minute technology can be used.
There is no need to worry if you do have children that are at the toilet training stage in life because manufacturers have designs so you can fit a seat over the top for them.

If style is something that “floats your boat”, an elongated toilet bowl will add a certain something to the overall look of your bathroom.

Disadvantage：

More expensive

Elongated toilet designs are more expensive than the traditional round shape, but that doesn’t mean you can’t find some great deals.
Take up more space
They will take up more space in your bathroom so bear this in mind if you have a smaller area to play with. (Actually, the difference in space is not huge, so unless you’re really tight on space, I’d recommend elongated.)