Is it better to buy a cell phone over there and purchase sim cards or buy a phone card thier. Base on price and conveyance.
It all depends on what you anticipate your need will be. Keep in mind that you can go online here in the States (ebay.com, tigerdirect.com, overstock.com, newegg.com, geeks.com, etc.) to buy an unlocked QUADBAND GSM mobile phone (GSM 1900/1800/850/900) to take with you for as little as $40 or so, then just buy a local SIM card once you arrive. SIM cards are usually easy to find right at the airport and they will help you activate it.
I have a BlackBerry with worldwide service (t-mobile) and I pay 99 cents per minute to use the feature. When I travel overseas, I also add on an email feature for $15 per month. That fee is pro-rated and can be switched on and off just for the time I'm away. In the grand scheme of things, the charges are reasonable.
When we travel abroad for only a week or two and don't anticipate making many calls - just a quick "hello" to the family back in the US to let them know that we've landed safely- we've found that the few dollars we waste by using our home cell phone are worth avoiding the hassle of tracking down a phone card or buying a new cell phone. If you are going to be making lots of phone calls home, however, a phone card will be the best way to go, because you don't want to rack up huge charges on your home plan. The option of purchasing a cell phone in the country you are visiting really only works best if you have local numbers that you plan on calling and you intend to so frequently. For example, when we lived in Europe for several months as students, all of the students bought a local cell phone. We needed it because the academic department and our local friends would need to get ahold of us. We had to call our family back in the States one time due to a family emergency, and seven minutes used up the entire 50 euro top-up card - probably the most expensive phone call we've ever had. I hope this helps - really, the best choice for you depends on which of these scenarios sounds most like your trip.