For a given line capacity, conventional reel is lighter and smaller than spinning reel.
For ease of casting, spinning is more forgiving than conventional because you can do quick cast with spinning while you must be more careful in performing smooth cast with conventinal.
For trolling, conventional is better because line won't twist as with spinning.
For drag adjustments, conventional is better because you just set drag once. With spinning, you have to set drag tight before doing a power cast or you might cut your finger, and you have to remember to reset the drag a little loosely. I don't like to use the Cannon even after wrapping the line around the captsan three times because I usually cannot put the cannon directly beneath the reel.
In terms of backlash, spinning is better unless you use conventional reels with magnets and/or brakes such as Penn 525 Mag, but you have to fine tune the conventional reel. It takes practice to master the conventional reel.
If reeling the line just above the rail, it is more comfortable to use the conventional reel because you keep banging the rail with the spinning handle.
For liveling, the conventional reel is better becasue you can put the reel in free spool and turning on the clicker.
Sometimes during the spinning cast, the line will snap because the bail close. I took care of the problem by remoning the bail from the larger reels.
The convention reel causes less casting pain than the spinning reel while making power casts unless you use glove finger, bike tube or money counting thimble for holding the spinning line.
In term of adjusting the drop length, you can set it precise using the conventional reel. It is a pain to adjust the drop length with a spinning reel.
If distance casting is not involved, the conventional reel is probably better.
If heavy weights are needed for casting, conventional reel is better. Spinning reel is more suitable for lighter weights.
If casting at night, spinning reel is easier to use.
Spinning reel is better if you plan to cast plugs a lot because you don't have to worry about backlash.
In case of fishing stress such as fish jumping all over the place, spinning is better as this is no time to get a backlash with the conventional reel.
Actually, it depends on the fishing situation, but it is impossible to come up with one reel type for all purposes. Why not use different reel types? That is the fun of fishing.
I grew up on conventional reels before the spinning reels came out. Now I use mostly spinning reels, but I am beginning to go back to conventional reels for some situations as listed above.