their both different tvs , the plasma will probably be better for watching sports , as plasmas deal with motion better than lcds , but the b550 is a good tv with great reviews , and will look a little sharper and punchier than your panny , the plasma has a more natural look , but then many prefer lcds anyway ...
its a case of finding out which you personally prefer , the b550 doesnt have a 100hz engine , which helps with motion , this is not a problem for some , but others feel they need it , its impossible to say how you will find the tv , you will need to demo against a 100hz tv , maybe the samsung 40b650 , and decide which suits you ...
if you think you want to stay with plasmas , id try and find a panny 42g10 , they are very good in my opinion , but are now discontinued , you may still find one though , just make sure their in stock if you see them advertised ...
or , you could wait 6 or 7 weeks for the new range of tvs to appear , seeing as you currently have a tv , and dont have to buy right now , thats what i would do ...