Is There a Big Difference Between 360-390 Implants and 390-420?

There is not a lot of difference between the implants at all.  The main thing is that the larger implant will have a slightly larger diameter by a few mm, and perhaps a slightly greater projection (go to the site and look at implant specifications).  Personally/professionally, going up one size without a good reason, doesn't seem worth it.

If you are looking for more projection, look into high profile implants.  For a 30" chest wall, your implants may give you too much fullness, especially laterally, the cleavage area, and even the upper pole.

The short answer is no. An Allergan medium profile style 68 390 cc implant is .3 cm wider and no difference in forward projection by measurement. 

In general, if the implant was sized properly relative to the breast width the first time, you can go up to a Moderate Plus Mentor or a high profile Allergan and add to the forward projection size by up to a half cup size. If the tissues have had a chance to soften and settle around the original implant and the capsule space is reasonable, you can change to and implant that is 1 to 1 1/2 cm wider and increase the breast size by about a half cup size at the base. The maximum is the do both an increase of the width and the profile. 

In changing to a 390 (filled to 420) cc implant from a 360 cc implant I doubt that you would feel it was worth the cost but it's not difficult to do if you already have an inframammary crease incision and the tissues have adjusted to the first implant.