Shape is the first filter worth applying, because it decides where the mass sits and everything downstream of that.
Round
The balance point sits low, close to your hand. The sweet spot is wide and central, which means an off-center hit still comes back over the net instead of dying on your strings.
What you notice on court: the racket gets to the ball. Defending off the back glass is comfortable, the volley is stable, and your arm is fresh at the end.
What you give up: free power. On the smash you supply the speed, the racket does not add much.
Buy it if you are in your first two years, you play mostly at the back, or you have had elbow or shoulder trouble.
Teardrop
Balance in the middle. This is the shape most club players settle on, and for good reason - it does nothing badly.
What you notice: enough weight up top to finish a bandeja with authority, enough control to reset a ball off the wall. The sweet spot is smaller than a round frame but still usable.
What you give up: it is not the most forgiving frame and not the hardest hitter. It sits between.
Buy it if you play both ends of the court and you are past the stage of mishitting regularly.
Diamond
Weight high in the head. This is a specialist tool.
What you notice: the smash lands heavier and flatter than anything else. From the net you finish points that a round frame would only extend.
What you give up: a lot. The sweet spot is small and sits high, so a late swing punishes you with a dead ball and a jarred arm. Defending is harder because the racket is slower to move.
Buy it if you already arrive on time, you attack from the net, and you are honest that you play there.
The mistake most people make
Diamond frames sell far beyond the number of players who should own one. They look like the professional choice, and pros do use them - because pros arrive early to every ball.
If you are choosing between a diamond you might grow into and a teardrop you can play today, take the teardrop. You cannot grow into timing you do not have yet.
How to check yourself
Next session, notice where on the face you actually strike the ball. If it is high and central most of the time, a diamond will reward you. If it moves around, stay with a wider sweet spot - the extra power of a diamond is worthless on a mishit.