WDs (Ducky & G'S) are WDs for the simple reasons that the label is cheaper, quicker and easier to produce than a multi-coloured one, in the early to mid sixties. At this stage the record is only being produced in small numbers to promote / demonstrate / audition the song usually by giving them to DJs to play on the radio stations. Therefore, it is all expenditure, and saving a couple of cents per unit is worth the while. If it was a success the record would be issued, and probably produced in runs of 500 plus, in the label's standard coloured format. Hence the reason why sometimes, some demos are rarer and more expensive
There are sometimes a "test press" of a record (Shufflers) kicking around which is one or two discs pressed just to check that the lacquer is OK
However, with the big successful labels like RCA, Columbia, I think they just automatically ran off a load of demos and issues in anticipation of a hit! Those that weren't became our "Northern Soul" and in these cases, the issues seem to be "rarer" than the demos (Jimmy Fraser). This could be because the demos had been distributed more widely before the plug was pulled on the song, and the issues just sat in a warehouse before being destroyed to make some more room.
If you lift up you box of records and see how heavy 300 records are, you realise why it was cheaper to send a real-to-real tape or lacquer to LA, NY, etc, rather than press the records up in say, Detroit and mail them out. Hence you may find different label designs (Jimmy Ruffin) and styrene on the West coast, vinyl on the East, but not exclusively.
In the 60's they seemed to like to have two tracks on a disc, I suppose that if the DJ/public didn't like the 'plug' side they might like the other! If they didn't have two songs, you might find the WD is either one sided, or has a "filler" on the B - check out Stanley Mitchell's 'Get it'!
Although LPs were being produced from the mid 60's in stereo, you don't really see stereo singles until to early 70's. I presume radio stations had better equipment and those broadcasting in FM could broadcast in stereo, but the choice between mono and stereo had to be still available. It is interesting that some companies (Atlantic) started to distinguish between the sides by putting a coloured label on the stereo side leaving the mono side a traditional white so as not to confuse the older DJs!
This has also led to the rarity of some 70's sounds as they are only available on the issue as the non-plug side as all those buying Ujima demo's have found out the hard way!
As for differences between demo and issued track, I don't think there are any if they've got the same catalogue number as they'll be pressed of the same lacquer. There a few records that where there are two demo's of different takes of the track, but only one went on to be issued e.g. Carl Burnette. Also, I think there may be a few spoken intros (on one side of the demo) missing from the issues, thank god!