All I can tell you is that the Paradigm's will have signigicantly better bass response than the PSB's. The Monitor 7's have 6.5" drivers while the 4T's have 5.25" drivers if I remember correctly. I listened to both of these speakers briefly when I was buying my system, and I preferred the Paradigm's. But, you should really try to listen to both of them for yourself, with the same electronics if possible.
BTW, I ended up with Paradigm Monitor 11's so my opinion might be slightly biased.
I've listened to both but never side by side and with quite a bit of time between auditions. I think both are excellent speakers and I recall the PSB being very favorably reviewed by Stereophile awhile back. Personally, I'll probably go with PSBs over the Paradigm but this is because I want bipolar surrounds. However, I'd be happy with either one. If you can audition both, do so. If not, just go with the Paradigms.
I've auditioned the Monitor 7 and 9s before and while they did put out quite a bit of bass, it was pretty boomy bass, and midrange wasn't very good, especially on the 9s due to their 8" midrange drivers. I didn't audition the Image series though despite they being on my audition list. After a long search, I ended up with the PSB Stratus Goldi's