I have the LF+ Pro+ and I'm a huge fan of it.
I never used a Mastermind GT, but looking at the videos and the manual, it seems to me that the LF+ has more possibilities.
It has 24 buttons (2 more than GT). Each button can have 2 functions, holding the button more than 0.5 second (adjustable) you access the second function. So you can easily state that you have 48 commands available.
Each IA-button can have 20 commands ON and 20 commands OFF (10 on the GT).
Totally compatible with the Axe-Fx: IA-states, names, scenes, tuner, tap-tempo, ...
Like the GT you have pages, but on the GT only 16, the LF+ 60 (but I admit who needs more than 16).
Aside the 2 functions per button, the LF+ outclasses the GT with the huge amount of 45 different commands.
MIDI Command (program change, control change, note on/off, ....)
Auto- Tap-Tempo
Change Page
Change IA-Map
Change Song
Delay (ms)
Mod EXP MIDI #
Resend Current EXP
Send Exp Value
Change Exp IA trigger
Mod EXP CC#
EXP Slot 2 CC#
EXP Slot 2 Invert
EXP Slot 2 MIDI
External Dev Sync
Force IA Color Change
Toggle Page Function #1/#2
Go Global
G-Tuner
IA OFF Trig
IA ON Trig
IA Resend
IA Toggle
IF IA is ON, Stop
IF IA is OFF, Stop
IF IA is ON, Skip
IF IA is OFF, Skip
IF Processing Trigger, Skip
Last Page
Last Preset
Activate MTC
Preset Trigger
Recall Stored Preset
Recall Stored Song #
Resend Initial IA-Initial States
Set color
Set Status #1 LED CLR
Step
Store Preset
Store Current Song #
Stop
Sysex Send
System Snapshot Restore
System SnapShot Store
Trigger 1st Bank Preset
It's probably a little harder to program due all the possibilities, but you can make it as easy as you want. Excellent editor is included.
And above all, it's $600 cheaper.
Of course this is all IMHO and YMMV.