I have a question however .... by "slug gun" do you mean a smooth bore, using a rifled slug or a rifled barrel using ammo specifically designed for rifles "shotgun" barrels? I'll will give my opinion on both set-ups.

Set ups requiring a rifled slug are seldom truly accurate enough to be effective past 100 yards or so. I have yet to see one that would group well at that distance, and the arc is significant for sure. It is not a matter of if it will "kill" a deer at or past 100 yards, it is a matter of hitting the kill zone every shot. So if that is your set up, no need to spend $$$ on a variable that goes past about 4X to 6X. Something like a 1.5-6 would be a good choice, as well as a 4X fixed.

If however you are using a rifled barrel and "slug" ammo specifically designed for use in these set ups, the effective range is , in general out to about 200 yards. If you can manage the arc of these heavy, slow moving bullets, you probably would be served well with something like a 2.5x8 or 3x9.

As far as make ... I seldom come right out and slam any manufacturer, and will not unless I have first hand experience .... I will not buy a Simmons, BSA or Tasco brand scope. I have had extremely poor experience with products from all three of these makers. In contrast, I have had super experience with the Ziess Conquest line and the Leupold Vari-X III line. To be honest, I should have ... because we are talking 3X to 5X times the retail price.

I'd suggest a Leupold Vari-X III in 1.5-5 X 20mm, Illuminated for the smooth bore.
For a rifled barrel set up, the Ziess Conquest variable in something like 3-9 x 40 mm should do great.