GFX35 has a newer charger firmware than the BL. If you already have a BL or planning on getting a used BL, you can send it into CEI for a firmware upgrade. The upgrade is $25 + shipping.
Some BLs have an upper limit of 3800MaH. There is also a bug when cycling depending on the version of the firmware you are using. Mostly that is a problem if you are using Sanyo 3600s or some really, really good GP3300s.
I believe that GFX35 is also a little more accurate and the menu layout is easier to work with. There are also a few more options while charging and the ability to change settings during the charge.
If you need more details, send an "Jim Bailey" <email@example.com>
I sent him a few emails and talked to him on the phone before I decided to purchase the GFX over the BL.
As an FYI for those that may not know, older T30 & T35s with firmware v4.5 or less, have a 3000MaH or 3800MaH upper limit which is calculated like this:
Divide the charge time by 3600 to convert to hours, then multiply that number by the charge rate converted to milliamps ex. 5 amps = 5000 ma.
Which means that you would need to repeak to get a good initial first charge.