tweecer and the eec tuner are almost the same. Actually, the tweecer software is a mirror image of the software on sctuner.net. Very similar tables, format, process, ect. ect... The eec software is not hard to use, but its not user friendly AT ALL. If you're just getting into it, its very confusing since everything is in a .text format. The eec tuner will work with sctuner.net software, however I think that the tweecer board uses some firware on their boards to make them work only with cal-edit software. I've been tuning my car for a few years and just started using a tweecer on my friends stang. We were able to get over a second off his et with just tuning.
IMO a tuner isn't 100% needed unless you have drivability issues or you plan to spend many, many many long hours in the garage and dyno in order to get the best possible tune.
As far as data logging, the tweecer RT tuner can do it, however, its only based for A9L and other family codes that are not SC related. The eec tuner does not have the ability to data logg. I run a wideband o2 set-up which also doubles as a data logger so I'm able to tune, make changes, log data and see realtime a/f ratio's so I can get the best, most safe power possible. Its been worth every penny.
You can get the SCT pro racer package that has a lot more parameters decoded in the ford computer. The drawback is that its expensive, and the additional parameters don't get you much further from a performance aspect.
A tuner is a powerful tool, however it can spell disaster when in the wrong hands. Do some more research, download trial software and get to know how to tune eec systems before making your choice.