Which SIM card should I use for my new iPhone?

So I have an iPhone 3, but I dropped it. Now the screen is cracked and so I got a new one. The new one came with a SIM card, but the one I used for the old phone still works fine. Which SIM card would be better to use?

Use your old sim, keep the new sim as a spare.

Use your old one saves the hassle of telling everyone you have a new number

There's no iPhone 3. 3G or 3GS. Either or is fine. Same purpose.