Good questions.

According to this source, MGS uses a 25 watt transmitter with the high-gain antenna to send data to the Earth.

Info for the Mars Odyssey TT&C transmitter was harder to locate. This link on the MO website just mentions that they use X-band for data transmission but no mention of the transmitter power. Even the excellent Encyclopedia Astronautica doesn't mention the transmitter power.

A little Google search found the manufacturer's product information PDF file, which states the X-band transmitter output power is 13 dBm. That should be about 19.95 watts. I also found the price sheet for the transmitter. Such a deal.

20-25 watts doesn't sound like a lot of transmitter power, but combine that with a good high-gain antenna on the spacecraft with the extremely high-gain antennas in the Deep Space Network and you can achieve some pretty good data rates even across millions of miles of space.