firstly, if you are buying a T4 then make sure that you get everything that you need. It isn't just a laptop and some of the ebay sellers are a bit deceptive about this.
You need:-
An XP laptop
The muliplexer box (ethernet socket and some large round multipin sockets)
The extension cable
Cable for engine ECU
Cable for SRS
Cable for 5AS (if an early car)
SRS adapter (goes between the extension cable and the SRS cable)
A bunch of T4 CDRoms.
Normally that lot will cost you the thick end of £1000 on ebay.
Also there is no tech support with a used T4.
I also personally believe that the T4 is more difficult thing to use, and due to its size is a pain, and it really needs a dedicated laptop.
pscan on the other hand will work on the laptop that you most likely already have and it won't interfere with other stuff that you have on the laptop, the interface is much smaller and you don't need a bunch of cables. It just has one. You can just leave it in your glovebox for when you need it.
I don't think with the T4 XP laptop that you are going to be logged onto the car in one window and on the Internet googling your problem in another window at the same time. With pscan that's absolutely possible.
On an MGF things that a T4 does that pscan doesn't:-
Actuation of engine ECU outputs, you can force solenoids and things to click: I am planning to do this on pscan but it's low on my list because customers don't seem bothered.
Coding in of brand new ECUs: With a T4 you could buy a brand new unused ECU from MG Rover and code it you your car. pscan can't do this, but then I would be amazed if you can find a brand new unused ECU anywhere.
Calibration of idle stepper: so far I have only done this on the MEMS2J (MGF VVC). The others will come eventually but again no one is currently asking (except for the Mini guys).
Configuration of Pektron: T4 can enable and disable things like front fogs on the late model MGF/TF with Pektron.
Configuration of 5AS: I think you can change some options with the T4 and pscan doesn't do that (yet).
On an MGF things that pscan does that a T4 doesn't:-
pscan can log live sensor data to a file and you can open it later in excel/libreoffice: I don't think that a T4 can do this. Rob Bell wrote an article on how useful this was to fault find a dodgy ABS sensor. If you log the wheel sensor speeds to disc and drive around until the warning light comes on you can see that one speed sensor was out of wack with the others.
I'm not 100% sure but you might be able to graph it onscreen with a T4 though.
LE500, MGTF 135, 85th Anniversary service light reset: pscan can do it, T4 cannot
Coding of cheap knock off Pektron fobs: if the checksum digit it wrong (and it often is) then T4 will not let you program it. pscan will
Non MGF things:-
If you have a Landrover then a T4 is far more capable at the moment, especially on things like the P38 or Disco 2. On Freelander 1s pscan isn't far behind now.
Also there are lot of ECUs on the 75/ZT that pscan doesn't yet cover, and also the custom things that people want like power mirrors and such.
On the other hand there are scenarios where pscan does better:-
It will talk to the R75/ZT260 engine ECU in Ford mode (it's a Ford ECU) and dump a lot more data out than T4.
It will talk to the more complex cars like the R75 when ECUs are missing, dead or swapped: The VIN number is embedded in things like the airbag ECU on a 75 and the T4 get upset about it if you swap it; pscan doesn't care.
Certain cars like the ZS180 never worked well on the T4. I think that MG Rover went bust before all the bugs were worked out; pscan works fine on this car.
I can support strange combinations like T-series engine swaps in ZR/ZS that are difficult with a T4.
obviously I am a bit biased, but I did recently speak to a garage in Netherlands that has a T4 and a pscan and they told me that they much prefer the pscan and now pick it up by default in preference to the T4, but there are occasional (but rare) scenarios when they still need the T4.
For my parting shot, the MGF is probably the best supported vehicle in pscan, it is the one vehicle on which I would say you really don't need a T4. This is because of Rob Bell's support and also the MGF register and the MGF community in general. They have really embraced pscan and helped me to develop it, and the result is the very high level of MGF coverage in pscan.
Equally if you have maybe a P38 Range Rover or Disco 2 as well as an MGF, or maybe a 75, and a line on a cheap T4 then that might be a better option.
