I went WCing for my socket A quite a while ago after trying multiple 'high end' air cooling options. The change netted me around a 12C drop in load temps. Keep in mind I was not using air cooling equivalent to the high end blocks now (XP120, Scythe, Ninja, etc).
If you want to get into watercooling to try out watercooling you will most probably find out its quite a bit of fun. Its also expensive.
If you want to get into watercooling specifically for performance factors only and dont push your system to extremes...and I dont mean mild overclocking...I mean EXTREME....then I would look into high end air cooling. Pricewise your entry level for some pretty danged good air cooling with fan will run ballpark $50 while for watercooling you will most probably be looking at a ballpark $150 starting tag to hit the same performance levels. Quality watercooling offers more 'top end' performance but to justify it you really need to be someone targetted at it....unless you are like me and simply like to tinker with your system
The main kicker to keep in mind (assuming you buy new) is that for most of the quality performance type pumps the cost of the pump alone is equal to or greater than a high end air cooling setup will cost....and doesnt include the block, rad/HC, tubing, fittings, clamps, and reservoir or T-line components. Since you are looking at using an Eheim 1048 I am guessing you are going for silence over performance. With that in mind any of the 3 blocks will work fine.
Now with all that said if you want to try out WCing I would look into the Storm or Apogee for top end. You can also find the MCW6000 series still available at about half the price of those and it still gives very nice performance as would the Danger Den TDX or RBX. I personally lean towards the MCW6000 as a great starting point if you are not looking for top end performance.
Warning: The Apogee while looking very nice performance wise seems to be having some quality control issues concerning 'cleaning' the block post fabrication. In short people have posted about having quite a few shavings/cuttings present so if you go this way take the block apart and carefully clean it. Metal shavings flowing through a loop are a bad thing.