The 1.4 GHz Intel Core i5 processor is faster than the Intel Core 2 Duo processor in your 2009 Mac mini. Processor clockspeed is becoming increasingly irrelevant nowadays as it does not really equate to actual processor performance. According to PassMark, the processor in the 2 GHz Mac mini from 2009 has an "Average CPU Mark" of 1315. In contrast, the new 1.4 GHz Mac mini (Late 2014) has a "Average CPU Mark" benchmark score of 3620. Intel has done a remarkable job to make their CPUs more energy efficient without compromising too much on processor performance.
I have the 1.4 GHz Mac mini myself and for non-heavy use, it works just fine. However, for running many applications at once, you need to consider the middle model. I run many apps at once and I am a web developer, so this model is a little too slow for me (I have a MacBook Air with a 1.3 GHz processor, so I thought the base model would be just fine, but clearly the SSD in the MacBook Air makes a big difference). As long as you don't run too much at once, the base model will do just fine. Also bear in mind all new Mac minis have soldered RAM, so get the amount of memory you will need now and into the future for however long you plan to keep your new Mac mini. 4 GB RAM might become constraining for you in the future if you plan to keep your Mac mini for 4 years or more.
The one thing that I would seriously rate the Mac mini down on is the fact it comes with a Serial ATA disk drive that spins at 5400 rpm. If you can afford it and you plan to buy your Mac mini from the Apple Online Store, consider upgrading to either an Apple Fusion Drive (that combines a mechanical hard drive and a Solid State Drive (SSD) into one) or an SSD on its own. Both of these options will result in even faster overall performance because SSDs are much, much faster than a 5400 rpm Serial ATA drive; however both BTO options are very expensive.
Whatever you choose, however, the new Mac minis announced last month are faster your 2 GHz Intel Core 2 Duo Mac mini from 2009.