Dobsonians differ less from each other than most telescope categories, because the design is simple and the mirror does most of the work. What separates a good one from a mediocre one is a short list of mechanical details.
The bearings and the balance
The tube should move smoothly in both axes with light pressure and stay where you leave it. Too much friction and fine positioning at high power is impossible; too little and the tube drifts. Adjustable friction, or at least a design that can be adjusted with a washer, is worth having.
The focuser
This is where budget dobsonians are cut. A plastic rack and pinion focuser with visible slop makes precise focus at high magnification a fight. A metal Crayford, especially a dual speed one, is the single upgrade most owners make and the one most worth paying for at purchase.
Solid tube, collapsible or truss
A solid tube is cheapest and most rigid. A collapsible one packs shorter for the same aperture and costs more. A truss breaks into two pieces and is how the largest instruments are made portable. Above 200mm the packing question usually decides which you should buy.
Where to buy
Sky-Watcher and Celestron both publish printed prices for dobsonians on their own stores, so the comparison is straightforward, and every specialist dealer stocks them. The second hand market is excellent for this design, because there is so little to go wrong and large instruments move slowly.
Questions people ask about telescope
What is the best dobsonian telescope?
The largest you will carry, with a metal focuser and adjustable bearings. Optically the designs differ far less than the mechanics do.
Where can I buy a dobsonian telescope?
Direct from Sky-Watcher or Celestron with printed prices, from any specialist dealer, or second hand, where the design holds up unusually well.
Are dobsonian telescopes good for beginners?
They are the usual recommendation. Nothing to align, nothing much to break, and the most aperture available at the price.