What is the difference between instant and filter coffee?
"Filter" coffee in general, is coffee beans that have been roasted and ground and is for use in coffee making machines. This makes 'fresh' coffee that may have a variety of qualities not found in instant - such as sweeter or more distinct flavours, etc. 

"Instant" coffee is normally a coffee that has already been made (they had the fun already!) and is dried (usually through freezing) into a dehydrated state, hence we just add water and get coffee. 

I would think that most people who have drunk proper (freshly made) coffee would prefer the flavour, quality and effects from caffeine, over that of instant. For me, instant is a back up, in case the real stuff runs out, but I feel always tastes a little woody and not as satisfying in terms of a caffeine hit. There are several coffees available now - Kenco Millicano is one - that offer both instant and some kind of coffee bean, but all I see is coffee grinds in the bottom of the cup, and not much difference in flavour.