if you play other spec than Assa i would upgrade shoulder 1st, as they are BiS for all 3 spec. if you only play Sin and never plan on using shoulder (you should they are BiS) then i would go for boots cause more Agi, it's probably a very small difference and upgrading goes fast if you play a lot.
for relic it looks like MA > master alchemist > toxic blades (ST only) > GW (better on cleave) > the new garrote one (with 4p)... then again you can sims it
https://www.raidbots.com/simbot .
One last thing is that with the new 4p, TB talent, and new trinket in a bit. Boots are starting to drop in value and the bis combo is looking to be bracer shoulder. MA are also probably going to be closer to other relic in terms value.