Now, if you ask me, i would say that it really depends on the person in question about what is his usage going to be, and first thing that has to be done is set the priorities right, so i am going to put it in terms of that only
1. If you are going to watch SD content for the most part ( by most part i mean 70 to 80%) then in that case try whichever set you want it will look like crap only (there are minute differences between different brands about how they show SD content, and TBH they look like crap no matter which brand it is), but now considering that almost all the DTH operator have ventured into HD, so for that you can try this at the store also, it is nearly impossible to find out the difference between full HD and HD ready when both are playing the same HD channel, it might be evident if you have your nose up against the screen of the display but at normal vieiwng ditance it is impossible to see any difference). Also since the HD content will be scaled at 720p or 1080i, atleast for now even in US no channel has started transmission on 1080p, so you can well imagine how long that is going to take for it to happen in INDIA, and by that time technology would have changed also, so it's good too look for a future reliable product but getting lost in the future is also not a good thing. So here the choice would be an HD ready set rather than a Full HD plasma, size will depend on your viewing distance, 42" is perfect for 6 to 8 feet viewing distance, and 50" is perfect for 9 - 11 feet of viewing distance.
2. Now if you are going to game on your ps3 etc for the most part, then there is no substitue for a full HD plasma, and as of now most of the games are being created for 1080p resolution, and also this is where you will really see the difference between HD ready and Full HD. 50PK550 has an input lag of 86ms, and i suppose 560 wouldn't be far behind as well ( i could be wrong about 560) but then if you are really a hardcore, die hard gamer i wouldn't look anywhere other than Panasonic. But if you are short on budget i would go for the 50PK550 and suck up with the input lag thing, cos frankly the extra 8 inches makes all the difference in the world, considering viewing ditance as well.
3. Now if you are going to mostly use the display for watching movies, and you have a blu ray player, or are looking to buy one in the near future (by near future i mean in 2 to 3 months time), or are thinking of buying a media player (WD TV, Xtreamer etc ), plus you are willing to shell out more than 1000 bucks to own a blu ray movie, or have a fast enough connection to download huge amounts of Data, only then would i recommend going for the Full HD plasma, but if you are thinking of running DVD movies then there is no point in spending extra bucks for something that is not going to be used, the 50PK550 handles BD playback at 24p like a charm, it's one of it's huge selling point, and also the difference for BD movie is clearly evident between HD ready and Full HD, so if DVD's are what you are looking at, go for the 42PJ560, if you think you can shell the extra bucks for that movie theatre effect at home then nothing beats a Full HD plasma
Now with that being said, i am going to tell you that you are not going to find any set which is perfect, hell even Kuro's or LED TV's that are priced in lakhs are not perfect, everyone has it's flaws, in the end it has to come down at what you are going to do with it, and that is why i didn't go in the pros and cons of these sets, or a comparison between these sets, because that will be a pointless thing to do, that is like the "kohlu kaa Baihl" thing, you are going to go round and round looking for that perfect display and in the end you will see how much time you have wasted for nothing at all, so first u need to clearly set out in your mind what you will be doing with this display, there is no point in paying extra for something that is never going to be used or is not going to make a difference while watching, also looking to far ahead in the future can be a bad thing also, u can loose sight of the present, so a balanced approach is what does it here, once you have done that half the job is done, you can now narrow your choice down based on your viewing habits and trust me the decision becomes way more easier once you have done that, because the priorities have been set right and now you only have to look what suits you the best, it's similar to buying shoes from somewhere, u can get the best shoes in the world and every brand available at your discretion, with all kind of different shoes like anklets for playing basketball, runners, joggers, spikes, studs etc, but what's the point in looking and inspecting them if they don't fit you or are out of your budget and don't serve your purpose, that is why first thing that you do when buying shoes is u state the purpose for which you require them, then u look for the size, and then you see what suits you the best in that particular shape and size.
P.S: Sorry for being so blunt in my approach, and i know you wanted to know about the pros and cons of these sets, but after reading your choices i remembered where i was a few months back, so all i am trying here is to tell you what happened after months of searching and debating about the pros and cons of every display without getting anywhere in the end, so i want you to avoid that months of time wastage and skip to the part where i realized what was needed to be done which i have already explained above (that is decide the purpose based on your usage, whether u want a Full HD or HD ready display, what size is best for you 42" or 50", for what purpose mostly is the TV going to be used, Gaming or movies, what kind of movies DVD, BD etc.), because quite frankly it's a never ending sea of technology out there, every set will have it's up and down side to it, firstly you have to find out what fits your needs and usage, and then look for what suits you the best among the shortlisted one's and once you have it in sight, don't go looking for other displays, cos quite frankly that's gonna throw you back to square one, once you have it shortlisted just go for it and after it's done grab a pizza or whatever sizzles your tongue, something to drink with it, some nice fluffy pillows, and just enjoy the experience.