The dryer decision that matters most is not capacity: it is how the moisture leaves. Vented dryers need a duct with a real route to the outside; ventless ones do not, and that changes where a laundry can go.
Electric dryers need a dedicated 240V circuit with the correct outlet type; gas dryers need a gas line plus a standard outlet. Swapping fuel type means new supply work, so in practice the existing hookup usually decides. Gas dryers typically cost more to buy and less to run.
Manufacturers specify a maximum equivalent duct length, and every elbow counts against it heavily. An over-long or convoluted run means longer cycles, a hotter machine and a lint fire risk. Rigid metal duct is far better than flexible foil, which sags, traps lint and is prohibited in many places.
Heat pump and condenser dryers extract moisture without a duct, which makes a laundry possible in an interior room or an apartment where ducting is impossible. They run cooler and use markedly less energy, and take longer per load. The condensate has to drain or be emptied from a reservoir.
A dryer stacked above a washer needs to be a compatible pair with the manufacturer's stacking kit. Depth and door swing also want checking against the washer below so both doors open usefully in the space you have.
Whatever the type, clean the lint filter every load and have the duct itself cleaned periodically, since restricted airflow is the leading cause of both poor drying and dryer fires.
More depth: front-load against top-load and when appliances actually go on sale.
A conventional dryer does, ducted to the outside within the maximum equivalent length the manufacturer specifies. Heat pump and condenser dryers do not: they condense the moisture instead and drain it away or collect it, which is what makes a laundry possible in an interior room or an apartment.
They use substantially less energy and can be installed where no duct exists, which is often the deciding factor. The trade is a longer cycle and a higher purchase price. If you have an easy vent route and dry large loads frequently, a conventional vented dryer remains simpler and faster.
Manufacturers publish a maximum equivalent length, and each bend subtracts a significant allowance from it, so a duct with several elbows can exceed the limit while being physically short. Use rigid metal duct rather than flexible foil, and keep the run as straight as the layout allows.