I've never tried either but I just got this response from Larry Hemphill when I asked what he was using:
"My unit is the Lowrance X-85 - the best! The unit must be 240x240 pixtals for a sharp picture. You REALLY get what you pay for when you buy a meter. Don't but a cheap one, you will be sorry."
The only difference between the x85 and x75 is that the x85 has more power. The only time you need that power is if you need to graph something well over 100 feet deep.
The only difference between the x75 and x65 is the type of backlight and the pixel count. Use the x65 and xoom in to 2x and you're essentially viewing at 320x320 pixels. Ok, so you can't view the entire water column as good, but if you put the x65 on a split screen zoom I doubt you'll miss much information.
So buy the x65 and use the extra money to get a water temp display.
All those extra pixels on the x75 sure are NICE though...