I've had good luck with both of the OP mentioned products. I used the gear lube in my Sportster a few times and the shifting was vastly improved over HD's, or any other's, conventional oil. Also the oil works good in regards to temperature lowering and engine smoothness. In a pinch, I would use the Mobil again. Right now, I use Amsoil and Redline in my two bikes, although I have 3 cases of VP Racing 15W-50 I will start using next season for my Sportster and I just bought a case of Redline 20W-60HD for the RK. I am trying the VP due to the low price I got it for and the Redline because it quiets down the motor ( I have a nice loud high-lift Woods cam). I also continue to experiment with the gearboxes in both bikes and will be trying Redline MT-90 in the Sporty, previously one of the fluids that gave the best shifting was the Mobil 75W-90. In the RK the Amsoil 75W-90 in the trans and Redline MTL in the primary delivers superior shifting characteristics, however, I am going to try the Redline Heavy Shockproof next in the trans.
The Mobil is probably the easiest of the three to find locally for most people. I am lucky and have an Amsoil distribution warehouse 5 minutes from my work and a Redline distributor 10 minutes away, so for me I just decide which product works "the best" and go get it. Pricewise, I think the Amsoil and Mobil are very close, if not the same. The Redline has just recently taken a big price jump and is now more expensive (last year I got it for $6.95/qt oil and $7.95/qt lube) and is now $9 for me. Since I only change the oil once per year, price really is not much of a consideration, I just want the best fluids.
Having said that, I doubt you can go wrong with the Mobil products you mentioned, they are some of the best out there. One thing I will mention though, is that you don't need a 20W-50 in the primary. If you are going to use it for ease of purchasing (only have to buy 2 fluids, instead of 3) then it will work fine, but really, the main purpose of the primary oil is to lubricate the chain, and even an ATF would work in that regards. The fluid must provide some other ancilliary functions, but if there wasn't a chain, there wouldn't be fluid there. I tried Amsoil's 10W-40 and now Redline's MTL and they both worked just fine. I may even try ATF in the future, but the MTL works fine. My idea is to get as thin as possible while still providing some lube for the chain. I would suggest, if you are going the 100% Mobil route, to use 10W-40 in the primary.
