I'm not to sure which is the fasteror better, and this question is so difficult to answer as you can never replicate the same conditions twice, except that you can compare speed from Magnet-mix or whatever it is called, but even this will not really say what will happen when you connect to other hosts.
What the difference is between a basic and a pro version is I am not really sure and I have yet to see a quantified answer to the question. I can say that last night I was using 4.9.23 basic and getting very good speeds both up and down.
If you install a new version it will generally wipe out your previous version but will continue toy use the old configurations unless you delete those manually.
You need to hide the old version. I use MacOSX and have 3 Drives, so I moved the old version (in its folder to another drive and renamed it for good measure) alternatively you could save it to CD or something similar.
I understand that, on windows, old versions are stored in some way on your computer including versions you may not have personally downloaded. I don't think this happens on Mac and I would not like to guarantee that this always happens. There are a couple of threads on this subject. (I have hidden away a version of 4.8.1 and I am not letting it go just yet
It always seems a good idea too get a basic version first to make sure it works before paying the pro-fee, although I do consider it a reasonable amount to pay for the continued development of Limewire ....