Many movies these days will come with the option of widescreen or full screen built in to the same disc... some won't, leaving you to decide which to spend cash on. Consider this: while a full screen version will fit the contours of an older T.V, it's at the cost of having been "formatted from its original version". This means that at times, parts of the picture may have been cut out or edited differently to make that full screen fit. The added cost is that this sometimes means altering your experience. This might be less of an issue with a comedy like dumb and dumber, but might make more of an impact with say, Star Wars. If you're not accustomed to the black bar lines of widescreen, it does take some getting used to, but it's the closer thing to the theatre experience the director envisioned for the movie you're about to watch.
Some of the fun about the movies is watching the special features. If that kind of thing isn't for you, then you'll have less to think about when selecting the copy to go home with. If special features are your thing, then consider how some of your favourite movies have been re-released time and time again under various different labels (like extreme edition, platinum edition, 20th anniversary edition, etc...). What's the difference?
Sometimes the picture quality and/or sound have improved. Sometimes there is claim that they have but really they haven't... at least not to an easily discernable or important degree. Beyond that, there are the inclusion and exclusion of special features. Take for example: Terminator 2. It has been released with an extreme edition, ultimate edition and skynet edition. Amongst these are some differences in features and interface. While many would consider the ultimate edition to be the most bang for your buck, there is also the alternative to buy all editions in one special edition terminator skull. The point is that if features is a criteria for movie purchase (especially if it's one of your favourites), then it might be worth searching the internet forums for consumer reviews on the various releases.
On the topic of special features, you should know that some local movie rental stores carry what's known as rental copies, adorning the label: rental copy, rental exclusive or some other indicator. Such a copy usually means that the movie has been stripped of the special features you would otherwise receive if you bought it new. This is mostly the case for DVD. What some have found of rental store Blu-ray copies is that where there ought to be extra discs for special features, there aren't any.
If getting the barebones edition doesn't bother you, it's less of a hassle. But if it does, a suggestion would be to check for if you're buying a rental copy. Also, it might be worth checking the back jacket for the number of discs included and then verifying to make sure that they are there.
With advancing technology and the push from VHS to DVD to Blu-ray (...and digital copies), your old favourites are getting a much needed overhaul. Some gems however, get missed. Have a hard time finding an obscure oldie at the store? It may be because it's only available on VHS or only available in another format in another region. Too, some DVD versions have not seen transfer to Blu-ray yet... some won't. Research here is key in obtaining that highly sought out piece of childhood nostalgia. As well, keeping an open mind to searching across video formats can mean discovering movie treasures you otherwise might have missed.
Sometimes, the little bits of information on the back jacket of a DVD are essential in preventing post purchase (or even rental) frustration. This applies to the wonderful world of foreign movies, dubs and subtitles.
There are great movies out there that aren't available with English dub (and some with dubbing that are still better to watch with subtitles). Expanding into other language flicks can be a great experience... but a rather annoying one if you are unknowingly thrown into it. Though I can't say I was fooled into thinking "Pan's Labyrinth" was an English ;language movie, I must admit that I did fall victim to picking up a subtitleless DVD version of "Amelie" .
Here, things could have been clarified by scanning the DVD back jacket, specifically in the small area near the bottom that reads "Languages".
For more tips on Movie Collecting, see also:
25 Tips To Expand Your Movie Collection