I have the decision between an 18ft go devil with twin 23hp longtails or a 16ft regular Jon boat with 40hp outboard. It would be used for fishing and hunting in southern Illinois. Why do people say not to get a longtail if you're going to fish with the boat?? I need some opinions on which to get.

Longtail takes up a lot of room. They are also a pain pulling up to the dock etc.

"Why do people say not to get a longtail if you're going to fish with the boat??"

I have fished in your area for many years, and several decades with (and without) longtail motors and outboards.
I honestly have never heard any bullshit difference in the effect of fishing with a mudmotor versus an outboard. {I have always used a powerful 45-55lb thrust 12v trolling motor with the longtails for reverse and slow speed maneuvering}.

IF you like going fast , from honey hole --- to honey hole, then an outboard is hard to beat for the top speeds in deeper stumpless and surface vegetation-less waters.

Dual longtails were once popular in Carlyle lake area with duck hunters to push a hugely heavy load of equipment in the upstream shallows & stump fields. Longtails shine best in shallow, murky, and stump filled slough and swamp areas cause they are simply air cooled motors that don't shear a prop too fast, or overheat from water pump failures. Longtails are my preference for bowfishing both day and night on the big river edges. Also, Longtails seem to aggravate the asian carp to jump higher.

In short , the dual GD set-up will be about 2/3 to 1/2 less than the top speed of the outboard.
It really depends on where you want to go to fish & hunt.
Longtails are like off-road atv's compared to outboards. Slow, somewhat primitive, but will take you places most outboards can't get too.
Longtailers will be like driving a tractor, slower and will get deep in remote places .

The average John Q. Fisherman in your area can get by with a good outboard (especially with a jackplate) if he sticks to the well known rivers and lakes.

Breast of luck in your pick!