Slug VS Pellet

Pellet slow down over the distance while a slug retains most of its velocity. Slugs in small bore air rifles are picking up pace because of the advancement in technologies and better materials to retain air pressure. I think that in year or 2 we will have a .22 gun which can shoot pellets at 12 fpe and slug over 100 fpe. 

I think you're a bit pie in the sky with your estimate, given the laws of physics and energy transfer but it's all good. The problem with slugs is that pellet guns were developed to shoot pellets. Because of this it can be difficult getting slugs to shoot in them as well as a pellet. Some of us have gotten lucky finding the right combo, most are struggling to get the slugs to shoot as well as pellets. This is in .177 and .22 of course.

Of course now that slugs in those sizes are available you may find more tuners converting pellets guns into slug shooters. Unless you have a unicorn gun like mine you will likely have to make/buy/tune a gun for one or the other.

As for advantages slug, when accurate from your gun will shoot flatter further down range and buck the wind far better than any pellet of the same size/weight.

Perhaps you should tell us which gun and caliber you are considering for a better focused answer. My .357 caliber airgun is about equally accurate at 50 yards. At a 100 yards, the slug is twice as accurate. The pellet is a JSB 80.1 grain, the slug is the 145 grain Nosler extreme. The slug launches slower, but retains a higher percentage of it's velocity over distance. The pellet starts out faster and flatter, but after about 50 yards, it drops more than the slug. Might I suggest you get a free copy of Hawke Chairgun4 and play with the values. It tends to be in the ballpark with all ammo where you have a good figure for the BC (Ballistic Coefficient) and weight in grains.