I’d say Harris County is considerably more conservative than Travis County. For example, Obama won Travis County by nearly 24% in 2012, while he only won Harris County by .08%—less than thousand vote margin in a county where over a million votes were cast. 2008 was similar—nearly a 30% margin for Obama in Travis, but only a 2% margin for Obama in Harris. Before 2008, Harris County hadn’t gone Democrat in a presidential election since 1964, while Travis County only went Republican twice during that time: Reagan in ‘84 and Bush in 2000.
You’re right about Houston. About half of Harris County’s population is in Houston proper. Houston proper is very liberal (with the exception of the wealthy and conservative River Oaks-Memorial corridor) while Harris County outside of Houston is very conservative.
Basically, Republicans in the suburbs tend to more than balance out Democrats in the city. That’s why the mayor of Houston is almost always a Democrat, while county-wide elections are normally won by Republicans. It has been trending Democrat, though. Democrats have been winning county-wide elections in the two Obama presidential election years, while Republicans continue to win county-wide elections in mid-term years. We’ll see if that continues without Obama on the ticket.