Which band should I get on my phone?

(US only) If you intend to use your phone only in the US, then get a dual band phone that has both 850 MHz and 1900 MHz.  A single band phone with only 1900 MHz will give almost as good coverage.

(Internationally only) If you intend to use your phone only internationally, then decide if you'll be using the phone in countries that use the international frequencies, or in countries that use the US frequencies, or in both.

If you only need to use the phone in countries with international frequencies, get a dual band 900/1800 MHz phone.  A single band phone with only 900 MHz will give reasonably good coverage, but most international phones these days have both bands.

If you need to use the phone in countries that also have the US 1900 MHz frequency, get a tri-band phone with 900/1800/1900 MHz.

(US and Internationally) If you intend to use your phone in the US and Internationally, two frequencies are musts - 900 and 1900 MHz.  The other two bands are nice to also have, with 1800 MHz typically useable in more foreign countries than 850 MHz would be in the US.  The best solution is to get a quad-band phone with all four frequencies.