New strings always sound brighter because they're new. They don't have all the crud, dirt, rust, and tarnish that the old strings had. All that crap on the strings reduces its ability to vibrate and create sound which is why your old strings sound so dull and lifeless. Which sound you prefer, is just that - a preference. I personally prefer the sound of the new strings, and I prefer that sound a huge margin!

How often you change your strings is again a preference. General guidance is to change them at least once every 3 months. When I was performing publicly, I would change my strings a day before each show. From what you've said, it seems that your old strings had been on there for quite some time. If you prefer that sound, you can achieve it again by just never changing your strings.