If you like the EVO 74 but think its a bit big you will for SURE love any version of the EVO 70. They are all very good and in the big picture rather similar. All the XTV version (07 and forwards) are a bit more refined and get a wider range by being smoother on the wave, particularly when entering a top turn. I don't know exactly why the 07 70 changed for 08. My very personal guess is that the 07 EVO 75 was considered a bit too radical and had to be changed for 08 and then this "spilled over" on the 70 which got a bit thicker rails to power it up a bit in slower waves. The 08 70 is a great board too and particularly if you're not that light and if you don't predominantly ride very good waves, the 08 might prove excellent. But when looking at the whole range for 09, I think is was a good decision to go back to the slightly more "high speed" feel of the 07. The 09 75 will easily cover up for those who want to play it more safe.
As for the ET74, yes, if i remember correctly it started its life as a twin fin version of the EVO 70 2008. But of course a whole lot of changes was made so it not so relevant what the starting point was.
My advice to you is to grab that good priced 2008 E70 and spend the rest of the money to go and get some good waves. It fun to discuss the nuances of equipment differences like this and the ET is indeed a very fun board which will do some things to your riding and the 09 E70 is indeed a refinement and so on and so forth. If you feel you need a new spark in your wave riding maybe the ET is the way to go. But on the other hand, if you would go to a custom board builder, show him your current board and say that you like it but just find it a bit too floaty, chances are he would build you the 08 EVO 70 (but charge much more). So in that sense it's the perfect board for you. Refinement at a bargain.