First of all... let me assure you that these are not stupid questions. As a matter of fact, I have to admire someone for putting this much thought in their rod purchase decisions.
You have some very good points and I think you have almost answered your own questions.
For the average caster, a longer rod will usually assist them in making longers casts. So... if you're gonna want to be making 40ft+ casts on a regular basis then an 8' rod may be the better choice. (however, I've seen Doug Macnair regularly make 40ft casts with a 5 ft rod) If this rod is going to be a multi-purpose rod for small streams and for larger waters then the longer rod would probably be in order. If you're gonna use this rod as a small stream only rod then the 7'6" rod will be fine... so would the 8' rod. I would personally never go shorter that 7'6"... but thats me.
Both the Echo 7'6" and the Elkhorn 8'0" are excellent rods. I like them both very much. I presently use an Elkhorn 2wt 8'0" rod for small streams and it serves me very well. But, like I said, my next small stream rod will be the Echo 3wt 7'6". I have tried this rod and I really like it.
Doug says, "G. Loomis Metolius as being the best 3-weight I've thrown". I should probably use his lead to upsale folks. Doug has a point... the G.Loomis FR963-3 StreamDance METOLIUS - Presentation 3wt - 8'0" - 3pc is a very sweet rod. It retails for $350 and thats a good price for a rod like this. It is a gorgeous rod and it casts like a dream. It is a joy to cast. I have a G.Loomis StreamDance 4wt that is pure casting bliss. The G.Loomis StreamDance and Metolius rods are my very best looking and best casting rods... period. If you have $350 to spend then yes... by all means this is certainly the way to go. If you're trying to soften the financial blow then you can still do very well for less than half the price. Thats where the Echo and Elkhorn rods come in. They are very nice looking rods and excellent casting rods. The average caster will likely never notice the difference between the casting of the Echo and Elkhorn with the G.Loomis.
I'm sure you've allready read my article on small stream rods. Its at Favorite number of rod pieces?
I hope this helps. Perhaps Doug will chime in and give you his thoughts.