I agree 100% it does depend on whether the jubilee sounds and plays better and you are right about yangisawa and yamaha's build quality,i have owned both and i sold my yani 990 alto to get my 6 and have never looked back.O.G - I still have the rally spec Berlingo van thanks and it has just flown through its MOT - It comes in handy when I or the other half need to do the recycling!
Back on topic:
IMHO It doesnt really matter how good your 6 is. What should matter is how much better the sax you wish to replace it with plays and feels.
If it plays, sounds and feels better then get rid of the 6 if it doesnt then dont get the Jubilee.
Regarding selmers' build quality - it may well have improved but is still way short of the build quality of Yangisawa and Yamaha. They also take 3-4 times longer to set up in the shop prior to going on sale.
Im not Selmer bashing here either - I play a selmer SA80 tenor and wouldnt want to part with it.