Be careful talking blue vs red as cowls could be swapped.
I run many vintage 6 mercs, I try to stay with the correct vintage so my decisions are based on that more than which one is "better". I run a 1958, 62, 63, 66, and 74 6's. and had a 69 and 57.
Electronic ignition was introduced in 66. It is points switched, in 67 it was hall effect switched, in about 73 the distributor design changed a bit but still hall effect switch. Switchboxes are all interchangeable.
There is no doubt that the motors became more fuel efficient as the years went on, lower units improved with stainless shafts. Later direct charge motors still have many new parts available, like oversize pistons, wire hanesses etc. The 73 up motors have better exhaust tuning, better lower units, the latest distributor etc. For these reasons, given the options you listed, correct year vintage not withstanding, I would go with a 73 and up 115 to about 77. This is the dist model and maybe less $$ for a good one. If you can score a later year 115 or 90 they are much stronger, have no distributor and have a better tilt design integrated into the transom bracket. You have more options becasue you can use a 20 inch motor. If you want the motor to "look correct" then put earlier decals on with a year correct faceplate. You could easily make a later model 115 look like a 69 1250. So unless you are a purist this would be a pretty stealth combo for your boat. In the 69/70/71/72 years many things changed on the exhaust side and interchange between powerheads, and midsections becomes a little more tricky between models and years. After 73 less intercahnge issues right up to the last year like 87 or so. I always thought a good stealth combo on an older 15 inch transom boat would be a Short Shaft mid from 73 up used on a late 80's 1150, decals of choice to match year desired.
My experience is the 1250 is a fuel hog, the distributor has a unique advance/retard mechanism, it is the largest, 99 cube, cross flow intake/exhaust motor they made and it looses efficiency, the 90 cube version (100 hp) is a much nicer running motor. I'd say the same with the 1100 cross flow, it is a one year wonder, 93 cube just puched out for a little extra hp but very little improvement over the 100 and more difficult to find pistons etc. in case a rebuild is needed.
The 1150's from 73 up (blue stripe in your vernacular) are smooth and torquey. It runs about as hard as a 1500 version of the same motor except right at the top end. The later 115 with no distributor run very close to the earlier 1500 but run much better over the whole rpm range, just a tad slower on top end. The non distributor 90's will run similar to the distributor 1150's. This is becasue of a hp measurement change taken from the flywheel (more ) to the prop (less). The 1500 is maxed out and can self destruct unless careful attention is paid to fuel, timing, proping and jetting. The 115 is far more tolerant.
All Mercs are great in my opinion but within that, some are easier to deal from a parts/interchange perspective and perform better over the whole rpm range and fuel use.
My two cents, I certainly don't know everything, just my observations.
Randy