You mentioned you have the E220 CDI - which model year is your car? and what sort of budget are you looking at? That helps us to see which generation of GS/LS you should be looking at.
Lexus runs on petrol only after a brief moment with the IS going down the diesel route for the 2nd Generation. Reason for this is that Lexus is all about low noise, vibration and harshness and diesels are the opposite i.e, all they offer is NOISE, VIBRATION and a lot of harshness.
Now you have the E220 CDI, I know what that is like i.e my wife had a C220 CDI which uses exactly the same engine and the basic unit is the same since 2003. Now these things are gruff and vibrate like crazy so expect any Lexus to be more refined than the Merc.
Lexus run on petrol because petrol engines are generally more reliable, have no DPF issues and are much cleaner to the environment in terms of emissions.
In terms of size, GS is the same size as a Merc E class, the LS is much larger. The GS should have a lot more kit than the E class as standard.
So pros and cons compared to the Merc -
pros - standard equipment, more refined,, smoothness, reliability, dealer customer care, arguably build quality is better on Lexus
cons - more expensive to run (depends on if you go hybrid or not)
indifferent - service prices are usually about the same as merc