Speed booster is useless. Basically it's like a squid server added onto your router for $20 more. It doesn't help near enough to even justify a $10 increase in price.
SRX on the other hand stands for Speed and Range expansion, and it is definately a good buy. SRX uses MIMO technology (Multiple Input Multiple Output), which is one of the key features of the upcoming 802.11n Wi-Fi specification.
Basically the theory is that the more antennas you put on the router the more signals you're going to send out. Over long distances parts of a signal are going to be lost. The more signals you send out, the more fragments of signals you're going to recieve. You can then piece together those fragments and make an entire signal out of them.
That's how MIMO technology was explained to me. To answer your question, yes the SRX routers are worth it. I haven't tried one yet, but I do know that the new linksys router that uses SRX also comes with a WPA2 security encryption option right out of the box. Hope that helps.