Skip the Experias. While the hardware is nothing to write home about, the software is a real letdown. It's buggy, it's still stuck on Android 2.1 Eclair (so it's extremely easy to fill up the internal memory, which is small - no App2SD support), and they have capacitive screens, but no multitouch (yowza)!
Now, when it comes to HTC models, Explorer is a bit more modern. It does have worse camera, but it also has better battery life, a newer processor (although, speed difference is highly questionable), and faster internet speeds. It has got a newer version of the HTC Sense. It doesn't have the Gorilla Glass display though (the Wildfire S does).
The biggest problem I have with the Explorer though, is it's abysmal memory. It has only 90MB left for apps! Even if you move all the apps to the SD card, some parts (and big ones too) always stay in this memory, so you can install a really small number of apps. A complete shame. It is a bit better phone than the Wildfire S, but that memory just makes it pointless in my book.