I cut and pasted this from the Tower web site. These are the specifications for both.
I have both and I like the Great planes product better in general. The Robart clamping arms are nicer than the Great planes product. It seems from other posts on incidence meters, it is about an even split for preference. They are both good instruments.

Greg

GREAT PLANES
NOTES FROM OUR TECH DEPARTMENT
This is the AccuPoint Laser Incidence Meter from Great Planes.
This High Precision Tool is used to find Accurate Angle Alignments on
Airplane Wings, Stabilizers or Engine Mounts.

FEATURES: Integrated use of laser beam pointer is much more precise and
trustworthy than the weighted pointers and bubble levels found in
other meters giving modelers pinpoint accuracy. (See COMMENTS)
The laser unit rests in a ball bearing supported yoke that eliminates
binding and adds to the meter's precision.
The big scale is easy to read from any angle, and measures incidence
down to 1/4 of a degree. Reads 0-10 positive and negative.
The grippers on the clamps are perfectly aligned to stay true over
time, and the opposed, large-radius thumb screws are easy to
access and use.
It is designed to be set on a flat surface to "zero it out" with a
knurled adjustment thumb wheel before use.
Special notch at the bottom of the scale allows it to be attached
to an engine crankshaft for checking the engine's thrust angle.
Fits wing chords upto 16"
Accommodates wider wings with an optional 36" long aluminum bar

INCLUDES: One Laser Pointer with Yoke Assembly (See COMMENTS)
One Magenta Red Aluminum Bar
One Incidence Meter Scale/Engine Thrust Mount
Two Clamping Jigs with Thumb Screws

REQUIRES: Assembling of the Laser Incidence Meter

SPECS: Bar Length Overall: 20"
Length Between Clamping Jigs: 16-1/4"
Scale Height: 8"

COMMENTS: Warning: This unit comes with a device that emits a laser light.
This can be harmful to eyes if looking directly into the light.
This along with other laser type equipment should be handled
carefully. NEVER LOOK INTO THE LIGHT BEAM, and never point the
laser towards anyone.
The AccuPoint can be used to check:
The wing at the root and tip, for warping.
That the stabilizer is aligned with the wing (thrust line).
That the wing and stabilizer are both level.
The engine's thrust line.
That the fuselage is level.
Helicopter blade pitch.

ROBART
This is an Incidence Meter is for Setting Proper Thrust, Wing and
Stabilizer Incidence on Model Airplanes or is Used to
Determine Why a Model is Not Flying Straight.
This Will Work On All Model Airplanes with Wing Chords of 15.25" or Less.
Robart offers an extension for large wings, but we don't carry it(see COMMENTS)

FEATURES: Magnified Window, Anti-Parallax, Jeweled Movement Scale Meter.
Reads Within 1/8th of One Degree.
Bearing Mounted Pointer With Mirrored Surface Behind For Easy Viewing
Steel Counterweight
Bubble Level
A Must For Building True and Straight Airframes
No-Slop "V-Block" Clamps

INCLUDES: One Model Incidence Meter
Two "V-Block" Clamps
One 18-1/16" Long Parallel Bar
One Instruction Booklet
REQUIRES: Assembly of Incidence Meter.

COMMENTS: Robart does make a 36" Parallel Bar part #404029M (we do not carry
it, but they will sell directly to the public).