Rega TT PSU..... one or two buttons ?

I'm thinking of buying the TT PSU as an upgrade for my Rega P3-24/Exact 2 front end. I've seen different photos on the web, and it seems as if there are two versions of this unit. One has one button (33 1/3 - 45 RPM), and the other has two buttons (Power and 33 1/3 - 45 RPM). Is there a difference ? Are there actually two versions of the TT PSU available ?
Also, what are the thoughts from those of you who have the TT PSU ? Is it a significant upgrade ? Thanks a lot.
Newer model has 2 buttons on front. All had 2 buttons. The older TTPSU has the power on the back. And yes, I had one when I had my P3/24 and it made a pretty substantial difference. Surprising really, Will depend on the resolution of the rest of your system, but do a web search you'll see people comments on this upgrade.
I have a P5 and the addition of the TT PSU improves the sound. To me, it is not substantial, but noticeable. I guess it is worth it if you can get one used. Again, every bit of improvement helps.
Cjnolan, I am having a tough time deciding between the basic P3-24 and one with the TT PSU and would appreciate if you can briefly describe what sort of differences have you heard. I don't mind if the improvements were subtle as long as they are noticeable but it would be disconcerting if I cannot hear any difference with the TT PSU on board.

On a separate matter, how would the P3-24 with TT PSU compare to the basic P5 without the TT PSU?
The TTPSU for my P7 has two buttons on the front - right one is power, left one id 33/45 rpm mode. Button out (red light) = 33; button in (green light) = 45.

Although P3-24 with TTPSU will be close to a basic P5, I would opt for the P5 and add TTPSU later as funds allow. I had a P5/TTPSU before uupgrading to my current P7.
Thanks for the response Miner42. Your response has somehow increased my chances of ending up with the P5 although the P3-24 with TT PSU was initially in my shortlist.
Okay................can somebody explain why the TT-PSU will make the P3-24 sound better ?? I don't want a theoretical answer, but rather a simple description of an audible improvement. My P3-24/Exact 2 sounds pretty incredible right now, through my Rogue Audio Cronus and Vandersteen 1C's.....what will the TT-PSU do to improve the music ??
TT PSU reduces motor vibration. Any upgrade like that will give the music a more relaxed, precise presentation. I use a P5 with TT-PSU and the difference is there but not huge. A quality cartridge will make a bigger difference IMO. In Canada the TT PSU was $400. I use an APC Back UPS which frankly makes as big an improvement and costs only $80 here. I would have spent it on a better cartridge first if I was to do it again. It does give you switchable speed so I do not regret getting it. Might make a bigger difference on your system than mine depending on its resolution.