Re: Ash or Alder?
Well, i'll try to answer this as good as i can.
Just remember that what i say is just my opinion, mmmmm'kay?
Ok...
About the tonal qualities, i would say that alder gives a tigther tone, while ash is more buttery, alder is more scooped while ash is midrangy, alder is airy sounding while ash is denser. And also alder has a more brilliant treble, while ash has a chimier top end.
Swamp ash (most common) are generally ligther or the same weigth as alder, while Hard Ash are about as heavy as maple.
About which wood is more similar to mahagony, would be like "what tastes more like pear= orange or watermelon". All three woods are completely different

.
Anyway, i would say ash, it's darker and more dense sounding, like mahagony.
I don't think that fender sells any teles with set necks and singlecoils. Anyway i would reccomend you to buy a Tele-style guitar from a different brand, like Schecter or similar. I think that when you buy a Fender you pay for the name

.
Anyway, good luck
-Erlend