MS is still in the FDA trial, so everyone has to apply to use it and fit their selection criteria (married parents, more children of the opposite gender from what you are sorting for, mom under age 40, and they exclude certain health conditions). It doesn't cost anything to apply, so I would encourage you to go ahead and apply soon. Better to get approved early than to have this application hold up your whole procedure. You do have to have a consultation before you do MS, and than costs $300, but you can do that closer to when you cycle even if you turn in the application now. You also have to have bloodwork done (for infectious diseases) and you might want to wait a while on that, since the results are only valid for one year. I think the wait time is shorter for MS/IVF than for MS/IUI, but you still have to go through the application process.
Using MS should give you a higher percentage of the desired gender embryos. It's not as high for boys as for girls, but it's around 70-80%, which is still a lot better than 50%. If you only end up with a few embryos, this could make a big difference. You just have to take the gamble on whether that difference is worth $3400. If you end up with a lot of embryos it might not matter, but if you only have a few it could be the difference between having something to transfer or cancelling your cycle, which would be money well spent. But of course you won't know that until after you have to make the decision. These high tech decisions are all so hard!