Hey there! That's a great question - you're thinking ahead about avoiding future headaches, which is smart! Beyond just I/O count, here are the key technical factors you should prioritize for your small packaging line:

1. Memory capacity - Packaging lines often need data logging for quality control and traceability. Make sure you have enough program memory AND data memory for future recipe storage, historical data, and any SCADA integration you might add later.

2. Communication capabilities - Built-in Ethernet is essential for connecting to HMIs, other machines, or factory networks. Check if they support industry standards like Ethernet/IP, Profinet, or Modbus TCP. The Siemens S7-1200 has great Profinet support, while Allen-Bradley excels with Ethernet/IP.

3. Expansion flexibility - How many expansion modules can each system handle? Can you add remote I/O stations? This is crucial if your packaging line grows or you need to add vision systems, barcode readers, or additional sensors.

4. Programming software & support - Siemens TIA Portal and Allen-Bradley Studio 5000 are powerful but have steeper learning curves. Mitsubishi's GX Works is often considered more beginner-friendly. Consider what's common in your region and what technical support is available locally.

5. Specialized packaging features - Look for built-in high-speed counters for encoders, motion control capabilities for servo drives, and support for packaging standards like PackML (especially important for Allen-Bradley in North America).

For a beginner, I'd recommend starting with whichever system has the best local support and training resources in your area. The Siemens S7-1200 offers great value and scalability, while Allen-Bradley CompactLogix is the North American standard with excellent packaging industry support. Mitsubishi FX5U is often more cost-effective but check if it meets all your communication needs.