different voltaje a/d sensors

[dhouston]

best choice?

That's too subjective a criteria.

You can maximize resolution by choosing Vref+ that is(are) slightly higher than the highest expected voltage(s) from your sensor(s). This would be an extension of Melanie's approach. For example, if the gyro never outputs more than 3V you could use 3V for Vref and power the thermistor circuit with 3V also. There's no additional switching.

The op amp solution is straightforward, time-honored and easy to understand. It can also maximize resolution and operates in realtime (essentially) so there are no additional delays.

Both increase noise sensitivity.

You really need to be clear about what you want. If you want maximum sensitivity, 5V/4096=1.22mV per unit while 3V/4096=0.73mV per unit so the op amp gives you a little more sensitivity. The difference is proportionally greater with 10-bit ADC.