Decide whether you need MES, ERP, or both before you take a demo
Electronics production has two separate software problems: running the shop floor (routing, work instructions, genealogy, machine connectivity) and running the business (quoting, BOM, purchasing, financials). Aegis FactoryLogix, Siemens Opcenter Execution, Cogiscan, iTAC.MOM.Suite, 42Q, Critical Manufacturing, and Eyelit are MES/MOM platforms. Cetec ERP, Epicor Kinetic, Infor CloudSuite Industrial, Plex, SYSPRO, QAD Adaptive ERP, Acumatica Manufacturing Edition, and Rootstock are ERP platforms with manufacturing modules. If you already run an ERP and need shop-floor traceability, you're shopping the first group. If you're replacing spreadsheets for quoting and inventory on a small PCBA line, you're shopping the second.
Match target company size to what's published, not to the pitch
The listings state a target size for each vendor: Cetec ERP is smb, several (Aegis, Cogiscan, 42Q, Eyelit) are mid, and Siemens Opcenter Execution, iTAC.MOM.Suite, and Critical Manufacturing are enterprise. A 20-person contract manufacturer evaluating an enterprise-targeted MES should expect an implementation scoped for a bigger operation, whatever the sales call promises.
Rule out deployment models you can't support
If you can't run on-prem infrastructure, cross off anything that doesn't document a cloud option. 42Q, Infor CloudSuite Industrial, Plex, and QAD Adaptive ERP are listed as cloud. Aegis FactoryLogix, Siemens Opcenter Execution, Cogiscan, iTAC.MOM.Suite, Critical Manufacturing, Eyelit, Epicor Kinetic, SYSPRO, Acumatica, and Rootstock's listings show hybrid or don't specify per-module deployment. TODO(editor): verify which of the hybrid-listed vendors actually offer a pure on-prem install versus hybrid-only, since some (Siemens Opcenter Execution) explicitly don't state a deployment model for the core execution product.
Ask who administers it day to day
A platform split into named modules (Opcenter APS, Opcenter Quality, Opcenter Logistics; or Aegis FactoryLogix plus Simio for simulation) means separate configuration and possibly separate contracts. If your plant has one IT person covering MES, ERP, and the label printer, a single-suite product with fewer named add-ons is less to administer, even if it does less out of the box. TODO(editor): confirm whether Simio requires a separate license from FactoryLogix or is bundled, per the listing's note that the two