From my many hours with a Sonic defender or two, I would place Ranged Defense higher than any other concerns. Ranged Defense does the best job of fixing one of Sonics biggest problems.
The funny thing about Sonic Dispersion is that it has a hole to sleep protection. This causes a lot of headaches for the Sonic/ defender since starting around level 40 almost every group seems to have some amount of sleep in their powers. This used to cause Dispersion itself to de-toggle most of the time when you needed it. The devs fixed that part, but I believe that Disruption field will still (I23 rules) drop but Dispersion is only suppressed.
Another thing you should know (in case you're not aware) is that the defender inherent works particularly well with Sonic/.
When you are solo, any added damage jumps -res powers more than you'd think.
When teaming your shielding keeps your team going well, but does allow in some consistent damage (unlike FF for instance) which will give you a huge discount to endurance cost of powers. When I say huge I mean many times I could run full toggles and blast non stop while not really seeing my end bar move.
Another thing to think about is that your pet is not very melee oriented. so you might consider that if you were wanting to place Disruption field on him for solo missions. I was researching this earlier and it seems that only the Leviathan pet will go into melee and then only after a few ranged blasts. Even with Lev, it's not really a melee pet.
Finally, recharge is always very nice, but only really useful for Liquefy and Vengeance in your build.
Vengeance will not be used all that much (outside of incarnate stuff) as you easily keep your teams alive.
Liquefy, due to when it was introduced into the game, has such a large recharge that it loses a bit if it's importance. You will never be able to get it's recharge times down to the level they should be (about 45 seconds, IMO). It's an "I win" button when you can use it on teams, but not quite as awesome as it could have been due to that 300 sec recharge.
by the way, the hold is almost useless in that power as it's just a chance to hold (like 2%) unless that was fixed at some point. If you want to use it as a mule, that's up to you. I slotted for To Hit De-buff personally.
Hope all of that helps.