A LockerGnome reader sent in the question, “I want to switch to the iPhone from a Blackberry and can’t decide between a $50 3GS or the $200 iPhone 4. What is your opinion?”

This is a tough one to answer, especially considering the iPhone 4 has become a bit long in the tooth and a new model could be on its way soon.

Based on the prices you mentioned, you’re looking down the barrel of a two-year contract with AT&T. This means that whatever you decide will be the phone you’re stuck with until your contract is up, or AT&T allows an early upgrade. Early upgrade privileges are rare, but they have happened with the iPhone before, so it’s possible. This means that by the end of your term, you’ll likely have an iPhone that is several generations behind the current model.

For some time during your contract, there may be apps that take advantage of a more powerful processor that you may not be able to use with the 3GS. An example of this are some of the newer games coming out that fly on the iPhone 4’s Apple A4 processor and crawl at best on the 3GS.

Screen resolution is another thing to consider when deciding between the two. Video is just clearer on the iPhone 4. This is another area where 3rd party developers are beginning to redo their apps to cater to a crisper screen with more pixels to decorate.

Photo By: Yutaka Tsutano

If you plan on using the iPhone for video recording, you might want to consider the difference in cameras as well. The 3GS sports a VGA camera capturing video at 640×480 while the iPhone 4 shoots video at 720p. Video editing tools for the iPhone are also better tuned for a faster processor.

A higher resolution still camera with a built-in flash is also included on the iPhone 4 while the 3GS comes up 2 megapixels short and lacks a basic flash.

One area where the 3GS has managed to hold a much better reputation than the iPhone 4 is in signal reliability. When the iPhone 4 first came out, tales of signal loss when holding the phone a certain way spread like wildfire. The signal loss appears to be greatly reduced by using a case. While it may not be a huge issue for you if you plan on using a protective case, it is something to consider if you prefer to use the iPhone without one.

At the end of the day, the most important thing to consider is which device you think you’ll still enjoy towards the end of your two-year contract. Mobile phones are a commitment, either by way of time or financial investment. You can switch out phones every year if you wanted, but that privilege comes at a significantly steeper price tag that makes the $150 difference between these two phones seem rather small.