Depends on what's important to you.

The 7995 has a detachable face, the 7998 has the Fantom face (faceplate
folds itself away inside the unit).  If you like to take your faceplate
with you for security, get the 7995.  If the 7998 is stolen it will work
for the thief as is.  Watching the fantom-face emerge and put itself
away is kind of cool, but eventually that folding mechanism will wear
out.

Display: the 7995 has a 3 color dot-matrix display - the 7998 has a B&W
display but it's much higher resolution.  Look at the close-up pictures
of each display on Crutchfield.com.

Key color: the 7995 buttons can be set to blue, green, or red.  The 7998
buttons can be either green or amber.

Faceplate: the 7995 has silver trim - the 7998 is black.

The 7995 has a clock - the 7998 has a clock and a calendar.  Upon
power-up it will display various holiday greetings and even wish you a
happy birthday if you like that sort of thing.

The 7998 has equalizer settings - the 7995 does not.  You can also make
I-Personalize CDRs for the 7998 which will load different configurations
of equalization and time-delay settings that you create, without having
to do it through the user controls.

Other than those items, the controls and features are pretty close.  I
have the 7998 and I like it very much but I'm sure I would have been
happy with the 7995 as well (except for the silver faceplate - doesn't
match my dash).