And how much would I be looking to spend on a nice monitor? I currently game on a 59'' 60hz 1080p plasma. I'm worried that moving to a smaller screen will ruin the gaming experience but it would be more comfortable gaming with a mouse and keyboard on a desk. Also, what would be the best screen size?
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You're probably looking around $150 for a good 23" 1080p monitor, which is the standard nowadays.
I can't really say which one is better because it is a matter of opinion. I love playing on my 54" plasma and mostly play RTS games on my 23" monitor. I find the large TV to be a much more immersive experience.
The biggest advantage of an LCD monitor is they will have less input lag than most TVs. It isn't that big of a deal if you aren't playing competitively.
As for screen size, it depends on the resolution you want. For 1080p, 23-24" is the standard size. If you get a 27" monitor, it would be best to get one with a 1440p resolution.
Basically, 1440p brings out a ridiculous amount of detail, especially on a 27in screen from 3 feet away. That being said, the VT50 has far superior color accuracy and shadow detail over any monitor on the market so I pretty much always play on it since I am able to see incredible shadow detail in games Splinter Cell, Tomb Raider, and Metro:Last Light which have lots of dark areas.
It really comes down to whether you want to see more detail from sitting up close and playing on a smaller screen or you want an overall better PQ in terms of color and black level. (Bearing you have a decent, newer TV).
*As for comfort, sitting back in a Lazy Boy > sitting at my desk. I sit at a desk 40+ hours a week.