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Hi Schmourghenne,
I'd have to very much agree with the advice that Steve offered.
Have to admit up front I'm a big fan of Newtonian reflectors that offer by far the best value for money.
Specifically regarding the questions you are looking to get answers to.
The true power of a telescope (assuming good optics) is the light gathering power. You can compare different apertures simply by comparing the square of the diameter. The central obstruction is negligible in assessing light gathering power.
4" = 16
5" = 25.
Therefore the 5" will produce images that are nearly 50% brighter than the 4" and means you can see objects 50% fainter. It will also have slightly better resolving power meaning you will get a little more detail in the image too.
However the 8" aperture suggested has a light gathering power of 64 -- 300% more light that the 4" and 250% more than the 5" and has about twice the resolving power.
An 8" dob doesn't generally come with goto but as pointed out, if fitted with an AgroNavis (the best DSS in the world - and Aus made), you can have push-to. A dob with push to is _dead easy_ to use. The Autostar and Nextstar systems are good but the Argo is even better -- wouldn't be without it.
Unless you have a midget car (assuming you get it fixed one day or another), the tube will fit across the back seat, and the mount in the boot.
As you have already recognised, it will need collimation from time-to-time. I know this can seem frightening, but it is like riding a bicycle. I know I'm an old-hand but I've got a truss-tube dob that is more demanding generally than a simple hard tube. Out of the ute after an hour riding the road it takes 2 mins at most to get spot on. The first time you do it it will take 20 minutes, the second time 10, the third time 5 mins etc etc. It isn't hard I promise you if you have the right tool (also cheap). Owning a newtonian means you should also remove and clean the mirror occasionally, but if kept capped it only needs to be done every 12 months. Remember a Schmidt corrector plate also needs occasional cleaning.
Owning a Schmidt will not relieve you of the need to collimate occasionally and collimating a Schmidt is somewhat harder and there are no tools to help. You have to do it on a star or an artificial star.
In terms of price, the 4" Celestron and the 8" dob with push-to will be comparable. The 5" will actually be a bit dearer than the dob. You don't get tracking with the dob, but you do get a vastly better view due to the increased aperture. Which is most important to you?
In portability, the 5" won't be easy to tote around all in one go -- your hands will be very full. The 8" is certainly a bit harder and bulkier, but with a sling for the tube you can carry the whole lot short distances without too much hassle.
Yes the dob is heavier and bulkier but is not real hard to move around. It will take up more room at home but if stored assembled with the tube upright and capped it will only take a foot square of floor-space somewhere. All catadioptric telescopes are prone to dew build-up on the corrector so counter measures need to be taken. Attacking this problem properly can be a mite expensive.
In the end, buying a telescope is a bit like buying a car. It is a personal choice and is made with your personal needs in mind. In my opinion you should carefully consider the advantages a 8" with push-to has to offer. It's has some very considerable advantages. But it is after all your choice.
Re this light-pollution business, it is an urban myth -- aperture rules.
Hope this is some help.
Best,
Les D
Contributing Editor
Australian Sky & Telescope
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