For the sake of others you intend to communicate with, please don't buy a Cobra 75... :snorkel:
Of all the radios I have difficulty in trying to understand the person behind the other mic on, the Cobra 75 is about the worst when it comes to transmitted audio quality (exceptionally muffled-sounding, something Cobra seems to have no interest in fixing either). I also cannot tell you how many times we've lost contact with someone using a 75 because they accidentally bumped one or more of the many controls that are crammed onto the mic without realizing it.
So I also would say to stick with a traditional-chassis radio.
One exceptionally good (and time-proven) unit I've found is the Uniden PRO-520XL. This is about the only sub-$80 radio I've seen in recent time that can rival the transmitter output quality of bigger radios such as a Cobra 148 (and it's actually sub-$50 to boot!). Obviously it lacks most of the features a bigger radio has (no SSB, noise blanker, built-in SWR, etc. etc. blah blah), but it is infact a very solidly built unit with good sound that will serve exceptionally well on the trail or highway.
http://www.amazon.com/Uniden-PRO520XL-40-Channel-CB-Radio/dp/B00004VXNI
Uniden also has a 510XL that deletes the RF Gain control and PA function for a few bucks less, but is otherwise is the same great quality radio (the 505XL seems much cheaper-made).
Agreed about antennas mounted up high on the roof. Transmitting antennas invariably work better when not partially blocked by the vehicle body.
It should also go without saying... the longer the antenna, the better. Right about 5 feet seems to be a performance tipping point, so you don't want an antenna shorter than this if range is important. As was mentioned above, a
102" quarter-wave whip does give the best range by far, however certainly there are situations where such a long antenna may be impractical. For these I like the
5.5' Francis CB-26 Hot Rod whips. They seem to have the best balance of performance with size and lighter weight.