Howdy Bowhunter3030.
Im going thru the same thought processes. Here's a couple things that come to mind:
1. A disc works really well when it is used after plowing. Therefore, you really need a disc AND a plow, in my opinion, to properly work the ground to about 8" deep.
2. discing bare ground is more difficult and usually takes several passes to cut it up. That's why plowing is often done first as opposed discing several times.
3. Most folks that have used a 3 pt disc, find that the 3pt style doesnt work the ground up the same way that a pull type disc does, at least from the folks Ive talked to. That may relate to the 3pt discs are lighter than the pull types or that the geometry of three points doesnt let the disc float as well as a one point pull type does. Think about that one...
4. A disc needs to be HEAVY to do a good job. A light weight disc will almost certainly need cement blocks or railroad ties added to get it to dig in deeper.
5. A tiller is more compact than a disc...A tiller is not very long, it is short...length wise, I mean. Some fellas use a tiller as ballast when using a front end loader! I wouldnt want a disc as ballast. It would extend out from the tractor too much.
6. Finding a used disc or tiller in my area is tough. It might be the same in your area. (And I live in an agricultural area). I do think the demand is higher, tho for a used tiller than a used disc. Translate: tillers are easier to sell.
7. A disc should be used at fairly fast ground speed. A tiller is used slowly-in a comparative way. So depending on your conditions, keep that in mind. For example, in a garden spot, high ground speed is NOT a real good thing. Likely there are fences or buildings close by and you cant turn your head around very quickly with a disc to see what you have done. A tiller is much more controllable in a small area. A disc works great in an open field. A tiller is better in a small space.
8. Tillers are more expensive. I suggest that the more expensive implements be kept under roof. A disc is often left outdoors. My point is, do you have the room to shed a tiller or disc? A tiller should be easier to shed, because it is so compact.
hope this helps.
dwight