The main difference between the Getaway and the glass boats is what they are designed for. The glass boats are designed to go fast and be raced. There are more sail and rigging features on them.
Even though you can load people up on th 16 and the 18 there is usually a marked performance difference on them when you do. The Getaway is designed to handle more weight and perform consistantly with different weights on board.
The Getaway, I think, is easier to maintain and can take more of a beating before it needs a repair. The glass boats are much cheaper if you buy used and are not looking at racing. They are also lighter (at least the 16 and 17 are) than the Getaway which makes them easier to drag around the beach.
It really depends on what your mission and budget are. If you can, find somewhere to rent at Getaway or someone to take you out on thiers. Then, you will know what you are comparing first hand.
In the market
Previous boats owned
'74 Pearson 30
'82 H18 Magnum
St. Louis, MO