The problem with google is that while it gives you a wonderful rundown of features, options, and sometimes spec comparisons, it's hard to get real user reviews of it. The closest I have come are the "pro" blog sites giving a review, but they often just do a cursory review that doesn't cover the day to day issues you come across with really using a phone. This is why i don't think this question is THAT bad of a question.
In a forum setting you are speaking directly to people that use the phone in a day to day situation, not a prototype given to them by the company that they have for a week or so. People on here are so quick to point to Google, basically saying "someone else has said this and I am pointing you to that someone else", but sometimes if we wanted the someone else, we would have just gone that way. People ask on here for a reason (sometimes not as it is a noob trait to not even look), so please don't be so quick to dismiss.
I was going to pick up an M600i before the self imposed phone buying freeze and I chose it over the E61 for mostly size and looks. On the UI front, I am a S60 fan and have been for a long time, but I like the changes made to UIQ3 (from what I can tell off the internet), and the only major concern is the available third party software. While phones come with all you "should" need, those programs are often missing small features that can be made up by third party packages. I think the lack of third party applications might be less of an issue as this new year rolls out as SE rolls out more UIQ phones to fill the market holes.
On the topic of memory for sony phones. The p990i uses duo which is up to 8gb (more than mini/microsd) with 4gb readily available (still more than mini / microsd). Memory stick costs are rivaling that of mini / micro sd so the costs associated with lots of memory is no longer a real deciding factor. M2 (which the m600i uses) is already at 2gb even though it's new and should reach the 4gb area in time for the release of new UIQ phones. UIQ has also proven that it can read the higher memory cards which is something of debate if the S60 phones can do that without FP1. From that hardware perspective, I think UIQ is really good.
Hardware combinations depend on the user's need. I would like a keyboard with an autofocus camera, and as it stands right now I can't get that with S60. I can't however get a "normal" sized camera smartphone with UIQ, so each platform (read manufacturer) has its own faults.
UIQ I think is a great compromise between the PDA functions of Windows Mobile (touchscreen wise) and the smartPHONE functions of S60. If you need just a bit more of a PDA feel to your phone/organizer then UIQ might be a good option (this is going off of Google, and is not necessarily true, this is where first hand accounts from users of the forum come in handy). I used to hate windows mobile, but as I have had to live with the Dash during this freeze, I have come to really appreciate the third party market to cover up the flaws in the OS. UIQ does not have this option, so the flaws here are a bit more glaring than S60 or WM5.
As stated before, its a YOU decision, and it comes down to what you need out of the device, but hopefully there are some people on the forum that can give a better first hand experience