Why do two enzymes with the same unit count perform differently?

Because the unit is defined by the supplier's assay. If one was measured on a different substrate, at a different pH or at 37 °C rather than 25 °C, the same nominal unit represents a different quantity of catalytic capability under your conditions. Comparing the assay definitions is the only way to compare the numbers meaningfully.

Should I buy on total units or specific activity?

Total units if you simply need the reaction to run and the preparation's other contents are harmless. Specific activity if the enzyme must be removed later, if carrier protein would interfere with detection, or if you are working at a scale where the non-enzyme content becomes a significant mass. For most analytical work specific activity is the more useful figure.

What happens if an enzyme arrives warm?

It depends on the form. Lyophilised powder is generally robust to a short excursion. A frozen solution that has thawed has probably lost activity and may have aggregated. A glycerol stock is intermediate. Rather than guess, assay a small quantity against a known standard before committing it to real samples — and specify a temperature indicator on the shipment so the question is answerable at all.

Enzymes, Inhibitors & Substrates — Enzymes and the Substrates and Inhibitors That Pair With Them

Specifying it

What decides the purchase

The compound is usually fixed by the route. These are the things that vary between suppliers and between lots.

A unit is defined by the assay behind it
One unit typically means the quantity converting one micromole of substrate per minute, but under conditions the supplier chose — a specific substrate, pH, temperature and buffer. Change any of those and the effective activity changes. When comparing suppliers, compare the assay conditions rather than the unit counts, or you are comparing two different measurements.
Specific activity tells you what else is in the vial
Total units say how much enzyme you have; specific activity, in units per milligram of protein, says how much of the material is the enzyme. A low specific activity means carrier protein and other species are present, which matters if they interfere downstream or if the enzyme must later be removed. For a purified application it is the more informative figure.
Storage form governs what arrives usable
Lyophilised powder, glycerol stock and frozen solution have different stabilities, different reconstitution requirements and different tolerance of a delayed shipment. A glycerol stock that arrives warm may be recoverable; a frozen solution that has thawed and refrozen usually is not. Match the form to the transit route as well as to the bench workflow.
Pair the substrate and inhibitor to the enzyme deliberately
Chromogenic and fluorogenic substrates differ in sensitivity, in detection wavelength and in whether the product inhibits the reaction. Inhibitors differ in reversibility and specificity. Ordering an enzyme without settling its substrate and inhibitor at the same time is how an assay ends up half-built with a lead time in the middle of it.

Questions

Why do two enzymes with the same unit count perform differently?: Because the unit is defined by the supplier's assay. If one was measured on a different substrate, at a different pH or at 37 °C rather than 25 °C, the same nominal unit represents a different quantity of catalytic capability under your conditions. Comparing the assay definitions is the only way to compare the numbers meaningfully.
Should I buy on total units or specific activity?: Total units if you simply need the reaction to run and the preparation's other contents are harmless. Specific activity if the enzyme must be removed later, if carrier protein would interfere with detection, or if you are working at a scale where the non-enzyme content becomes a significant mass. For most analytical work specific activity is the more useful figure.
What happens if an enzyme arrives warm?: It depends on the form. Lyophilised powder is generally robust to a short excursion. A frozen solution that has thawed has probably lost activity and may have aggregated. A glycerol stock is intermediate. Rather than guess, assay a small quantity against a known standard before committing it to real samples — and specify a temperature indicator on the shipment so the question is answerable at all.

Enzymes, Inhibitors & Substrates

What decides the purchase

A unit is defined by the assay behind it

Specific activity tells you what else is in the vial

Storage form governs what arrives usable

Pair the substrate and inhibitor to the enzyme deliberately