Go currently have 14 Boeing 737-300's, with more on the way.
The fleet is made up of aircraft with, I believe, 2 different types of engines.
The older and less powerful aircraft are, I believe, G-IGOA - G-IGOH. The newer ones are G-IGOI - G-IGOS. (I may have got the division in the wrong place slightly)
The later aircraft are more powerful and offer better economy, and are therefore used on the longer routes, and especially those which are "hot and high" (e.g. Malaga in the summer with the high surrounding terrain)
Having said that, this is a rule of thumb, and exceptions are very frequently made in order to get the best usage of aircraft possible.
Naturally, the older aircraft are seen more frequently on the domestic routes such as Edinburgh, Glasgow and Belfast. Whilst the newer ones tend to get the Malaga's, Naples, Faro's, and Tenerifes etc!
Outwardly - and from a passenger point of view, there are no difference between the aircraft, apart from that they are all painted different colours!
On a typical July Venice trip, you can pretty much expect a newer aircraft on the run.
Hope this helps!
FJW.