A long focal length doesn't offer large advantages.
In the days of yore, with traditional eyepiece designs the long focal length allowed the use of longer focal length eyepieces
to reach equivalent magnifications over a short focal length design. Today modern eyepiece designs
can offer a short focal length with a wide field of view and long eye relief.
The difference in focal length between an 8 inch dob (1200) and a 127 MCT (1500) is not that much.
Here are some of the considerations between the scopes you listed.
If the 8 inch dob has a two inch focuser it can reach a true field of view of 2.25 degrees. The MCT will be limited
to around 1 degree.
The MCT since it is on a goto mount , it will be able to find and track objects. This can be a big help in light pollution.
Finding dim targets is hard. Sometimes no guide stars are available for star hopping.
However goto is not magic. There is a learning process in setting up the goto. There is an alignment procedure that must be done each time the scope is used.
The SLT mount cannot be used manually, you always have to use the hand box to move scope. It takes batteries.
You will probably have to buy some larger high capacity battery if you want a long viewing time especially if it is cold.
If you have no batteries you have a very pretty paper weight.
Even though much smaller than the dob, you will probably find yourself making two trips to setup the MCT too.
The tripod and scope is a bit awkward to carry. I once had a telescope pop out of the vixen saddle, I don't carry two as
MCTs are susceptible to dew and frost. if you live in a humid climate you made need a dew shield or a dew heater.
The dob will have to be collimated every once in awhile. This is an easy process but once again takes learning to
do it well and accurately. The MCT if handled carefully will never need collimation.
The dob has a lot more light gathering power and resolution. Just about every target will be better in it.