There's often confusion regarding ketones and meth manufacture.
P-2-P; aka phenyl -2- propanone or benzyl methyl ketone
While the P-2-P ketone route to meth is more complex than the usual reduction processes employed for pseudo, an experienced chemist would normally prefer this over the pseudo route if he was after the purest product (and not the most money for the least time). A route that employs a ketone intermediate can allow for better purification.
BUT, although meth is meth, not all Meths are equal. The end products from each procedure are distinctly different.
Different in the sense that if each process resulted in a theoretical 100% yield, the ketone route product would only contain half of the more active dextro-rotatory isomer, where-as the pseudo product is 100% dextro-rotatory (d) meth. With ephedrine or pseudo, the amine is already attached to meth's chiral (second alkyl) carbon and so the configuration is unchanged during synthesis.
With the ketone routes however, the chiral carbon is turned into an essentially flat molecule (planar) at the ketone stage. 'Adding' the amine (reductive amination) results in the chiral carbon-nitrogen bond orientating itself so that the 'methylamine tail ' of the molecule either sticks out one side or the other.
This occurs because of the angles of available attack. The amine can approach from either side and so has a 50-50 chance of attacking from the front or back of the C=O (ketone carbonyl group) and results in a 50:50 dextro- levo product.
Separation requires a pure isomer of tartaric acid or similar resolving agent together with some tricky washing. The alternatives using chromatography are time consuming so it's unlikely this isomer separation would be done for street gear.
Don't get P-2-P confused with MBK or acetone. All 3 are ketones and all three can be used in meth manufacture. MBK can be used to wash or crystallize the final product. It has a characteristic ketone odour. MBK is included in many paint thinner preparations, also often used as a source of acetone.
Because the ketone routes are much more involved than the more simple pseudo processes, some chemical understanding is essential. Often impurities are present at the ketone stage. These need to be removed by distillation and purity checked using a bisulphite adduct or similar method.
Benzaldehyde was once often used as a starting material for the ketone route. Other routes went via benzaldehyde but started from toluene, benzyl alcohol or benzoic acid. Other methods include starting from chlorobenzene, or using the synthetic tools found in cyanide compounds, nitro-oxime routes and Grignard chemistry.
To be honest if you know what you're doing, meth can be made from almost any simple starting material which contains an aromatic ring. And there are just as many possibilities starting with completely benign materials that build up the ring.
Restricting precursor and staring materials, while in some ways necessary, always tends to breed improvisation. The "rough" tools of organic chemistry are the indispensable solvents, cleaning agents, and food products found everywhere. What it means for the end user is that the product potentially contains impurities from each of these extra steps and ingredient preparations.
Oh, and to address your second question; it should not matter at all if both products are relatively pure.
![:\](https://bluelight.org/xf/BL_Images/Orig_Emoji/wrygrin.gif "wry grin :\")