LNB cable of different impedance

A 300 foot cable run makes cable loss a critical factor, and mismatched impedance cannot help but add to that loss. I'm not familiar with your specific equipment, but it seems to me that you need not limit yourself to just two choices. Two choices you've not mentioned include:
1. switching to a 50 ohm LNB
2. a 50/75 balun

Once mismatched impedence is eliminated from the equation, cable selection is then dependent upon hardware signal requirements. Compare the LNB rated output level with the modem input level requirement. Armed with that range, you should then be able to go through the cable tech specs. Manufacturers of quality cable are not afraid to publish cable loss specs openly. Once armed with your own "acceptable loss over 300 feet" numbers, you can then select the most reasonably priced cable and connectors for your specific application.

In addition to signal loss, DC loss over 300 feet might be another critical number for you to consider. That's on both conductor AND shield.

//greg//