short -vs- long focal length

What's the difference between the two in terms of magnification and better viewing?

Lots of variables here. Longer focal lengths allow the use of longer focal length eyepieces with greater eye relief to be used, more comfortable for high power viewing. Shorter focal lengths allow for wider fields of views, bigger patches of the sky at a time.

Normally longer focal ratio optics are easier to fabricate because the tolerances are less critical. But sometimes the primary mirror is actually a very fast focal ratio so one can have a long effective focal ratio but with tight tolerances. SCTs normally use F2 spherical mirrors for the primary for example.

But in general, short focal length optics have more aberations, geometric and in the case of refractors, chromatic. That is why you can by a 120mm Achromat that provides decent views for about $350 but a well corrected 120mm Refractor will be an APO and cost more than 10 times as much.

Short focal ratio scopes are harder on eyepieces. An F10 scope can use about any eyepiece but the light cone of an F4 or even F6 scope will require better eyepieces to get clean widefield views.

When dealing with scopes of equivilent aperture that are not CATs, (SCTs and MAKs), the primary disadvantage of a long focal ratio/focal length is that the OTA is long, a 10 inch F7 DOB will certainly provide stunning views with nice sharp edges but the OTA will be about 70 inches long and weigh maybe 70lbs. This will also tax the mount, if the scope is to mounted Equatorially, it will take quite an expensive mount.

On the other hand, a 12.5 inch F4.1 Newtonian will require a Paracorr to provide decent views will be brighter by about 50% than the 10 inch scope and will need an OTA only about 50 inches long.

Jon