If you struggle with your irons, go for as forgiving an iron as you can get. These are called cavity back irons, and they benefit all golfers. With perimeter weighting the club twists less during impact so more energy is transferred to the ball.
There are a few other clubs which maximise their use of weighting to benefit players with a slice. If you tend to slice the ball, look out for clubs which promote a draw.
You should only consider getting blades or muscle back irons if you are a low handicap player. These irons feel great when the ball is struck from the “sweetspot” and provide great feedback and workability; however they are extremely unforgiving and can even be painful when the ball is struck away from the sweetspot.
If you buy blades, you should try them out first and look at them as a long term investment. Spend your money wisely as these irons can be quite pricey. Forged irons are more expensive due to the manufacturing process but they are definitely worth it for blades.
The average golfer (mid to high handicap) should look for a large amount of cavity back and a slightly oversized face. This will give the forgiveness required without compromising on feel or distance.
An iron with a small cavity and little offset will allow you to work the ball from left to right and right to left. This is great if you are starting to lower your handicap and want the freedom to manipulate the ball.