Buying new cranks?

2, 2.5, 3 Piece
This spec refers to the amount of Pieces required to assemble the Cranks.
3pc (2x Crank Arms & 1x Spindle) is the Industry Standard, and 2 / 2.5 are functionally the same.
2pc feature the 3pc design, but with one of the Crank Arms welded on to the Spindle.
2.5pc is similar to 2pc, except the Crank Arm can be removed from the other side of the Spindle.
All 3 of the options above are drilled for 9/16″ Pedals.
Avoid 1-pc Cranks at all costs. They are significantly weaker and do not fit most modern Bottom Brackets.

Spindle Diameter (19, 22, & 24mm)
The Spindle Diameter is the thickness of the Cranks’ “axle” that runs through the Bottom Bracket of the Frame. This spec is solely personal preference, and doesn’t effect how the bike maneuvers. Sizes include 19mm, 22mm, and 24mm (15/16″.)
Generally speaking, larger spindles are stiffer & lighter, without sacrificing strength in the Cranks. However, with a larger spindle means smaller bearings, which are more prone to blowing out.
(WARNING: I do not recommend running Spline-Drive [SD] Sprockets with 22mm or 24mm set ups, as there is different styles of splines that effect whether or not the cranks and sprocket are compatible. The chances of you getting an appropriate SD Sprocket for your cranks, without prior knowledge, is slim.)

Crank Arm Length (160-180mm)
The Crank Arm Length is the measurement regarding the distance from the Spindle Boss to the Pedal Boss. This does effect how the bike handles. Sizes for 20″+ BMX Bikes range from 160mm to 180mm.
The shorter your Crank Arms are, the easier spin tricks will be since your feet are closer together. Having shorter arms also clears a bit of room for your feet, so you’re not as likely to hit your heel on your rear peg. Shorter cranks are known to also feel stiffer than their longer counterparts. 
The longer your Crank Arms are, the easier it will be to pedal, as you have more leverage over your Drive Train. They are also renound for making the bike feel more stable, and less twitchy.

Spindle Interface
The Spindle Interface is the way the Crank Arm(s) connect to the Spindle. This can range from 6-Spline to 48-Spline set ups, to even Splineless Interfaces, such as Primo’s square-shaped Spindle & Odyssey’s patented Socket Interface (nonagon-shaped Spindle.)
As far as buying aftermarket Cranks goes, you will want to avoid 6 / 8 / 10 / 16 Spline set ups. They are generally cheaply made and are prone to failing at the split Spindle Boss.
Majority of quality aftermarket Cranks are 48-Spline. They allow for the Crank Arms to fit snug on the Spindle without the help of any unnecessary bolts or a split Spindle Boss. This is your best option for Splined Cranks.
There is also a few different Splineless options. #1 being Odyssey’s patented Socket Interface, which features a wedge system that is guaranteed to stay snug on the Spindle and never wear itself loose. You cannot run Spline Drive with Splineless Cranks.

Tubular / Square-shaped Crank Arms
This is another feature that doesn’t effect how the bike rides & is solely rider preference. 
Most people are fine with a simple Tubular Crankset, meaning the Crank Arms are round in shape.
However, a lot of riders who do Crank Arm Grinds prefer Square-shaped Crank Arms for their flat surface that is optimal for locking into grinds.

Bolt-Drive or Bossless
These are two different styles of Cranks for different Sprocket compatibility.
Bolt-Drive Cranks are the most common kind of Cranks, with a boss for a Sprocket Bolt located on the back of the Crank Arm(s). You can also run Spline Drive on Bolt-Drive Cranks, so long as they are 48-Spline.
Some companies also utilize the Bossless design, meaning there is no Sprocket Bolt Boss on the Crank Arms, limiting you to Spline Drive/Socket Drive Sprockets. Some riders opt for Bossless Cranks for the weight savings & clean look.

Heat Treatment
A lot of quality aftermarket Cranks are Heat Treated, which means the tubes have been heated and cooled in a way that strengthens them. 
The first and cheaper method is to use heat treated tubes to build the cranks. However, the heat of welding can negate the treatment of the tube near the weld, which brings me to method #2..
Post-weld Heat Treatment is a process that consists of heat treating the Cranks post-production so that the properties of the metal are the same through out the welds. This is the most efficient way to strengthen both the tubes and the welds. 


With all of this information & your own preference taken into consideration, you should be ready to pick out your own new BMX Cranks!

Info taken from: https://www.vitalbmx.com/forums/General-BMX-Talk,2/BMX-Crank-Buying-Guide,1318289