Singles rackets tend to sacrifice speed/maneuverability for power. Doubles rackets tend to be quicker due to the faster nature of a doubles game. Keep in mind that most manufacturers don't specifically categorize one racket as being for singles or doubles. Rather, they'll list the characteristics of the racket (balance, weight, flex) and what type of player they might be suitable for. Some marketing strategies do come into play regarding these player/style classifications. Basically, just find a racket that you think feels good and that should be it. When pros are asked why they use their current racket, they just say that it feels good. I don't think they care about what technology is going into the racket but just how it performs. An example where a commonly labeled doubles racket is used for singles is the Yonex Nanospeed 9000. This is a headlight racket that is used by many doubles players, but also by some of the top women's singles players.
it also somewhat boil down to personal preference. when i play singles, i am always out of place, whether on a rear court overhead shot or reaching to the net. in those occasions, i prefer to have a evenly balanced racket to give me the extra leverage. when i play doubles, i have pretty good defense but i am weak in smash, i prefer to have a more solid and heavier racket for the offense.
My personal preference is having 287 Balance Point and add high tension to make it more head heavy for doubles and singles. I like to use a slight heaver racquet maybe +5 more grams for singles. In my case, I use Pro tech Crossfire in Singles (84 grams) and Karakal Mtec 70 for doubles (76 grams). But for some reason, I can also use Karakal Mtec 70 for singles too since I smash much harder with it. Its sweet spot is pretty huge. I have NS9k that I broke twice from high string tension, its sweet spot once smash correctly with a lot of muscle fiber into it is faster than a blink of the eye. Unfortunately I've only done that once and thats one of the time I cripple the frame in two and had it replaced for 30$... Sigh... I'm still experimenting finding a suitable head heavy and light doubles racquet.
That's true it also depends on what type of matches/games you are playing and who your opponents are. If the doubles games usually involve a lot of quick exchanges, you might find yourself more inclined to get a faster moving racket if you find yourself struggling with a heavier and slower racket. If you prefer to move towards the back court and your partner likes to set you up for the attack, it makes perfect sense to have a racket that you can smash harder with (they generally tend to be more head heavy and solid, but to each his own). In reality, proper technique can influence your game much more than a racket, especially at lower levels of play. For instance, some people will grip a head heavy racket at the bottom of the grip when they're playing at the net in doubles or doing quick drives. Instead of changing rackets, they could grip the racket higher up and thus be able to swing the racket more quickly while also having more power in the back court.
Most smashing singles players (pretty good ones) generally use slightly head heavy to head heavy rackets while doubles players (the pretty good ones as well) tends to gravitate to the more balanced or slightly head light rackets. Flex on the other hand is the X factor, be it singles or doubles play, the preference isn't clearcut; from the pros to the rec players.
It all comes to personal preference. Generally the game in doubles are much faster. If you look at the top players in doubles, the prefer to hold the racket very high on the grip. Yonex introduces C.S. Cap for this reason to maximize the grip at this point. The Armortec 800 and 900 series has a longer grip also for this reason (Not a longer racket!). This would be the only part which can be defined as designed for doubles, but you can play with it as a singles racket also. If I play single, I hold the racket very low, just to extend the reach a little, but there is no part of a racket which has been designed for single play.
I recommend a slightly more aggressive (harder, head-heavy-balanced) racket for double since normal player i see can only be well-prepared to smash hard in double games, that kind of rackets can give you strength to end the rally with one smash.
