Hey there! I totally get your dilemma - inheriting a 90s-era S7-300 system is like being handed a classic car that still runs but needs some serious modernization. Let me break down the real cost-benefit analysis for you.

First, the S7-1500 upgrade path offers some compelling benefits: 10-50x faster processing speeds, integrated diagnostics that can reduce maintenance costs by 30-50%, remote monitoring capabilities (you can check status from your phone!), and future-proofing for Industry 4.0. The migration tools are actually pretty good - Siemens provides conversion guides that help preserve your existing logic. The upfront cost is significant (hardware, software, engineering time), but you'll see ROI through reduced downtime, lower maintenance costs, and improved productivity.

Now, the hybrid approach is interesting too. You keep your S7-300 running (avoiding immediate replacement costs) and add modern HMIs with edge computing. This gives you better visualization, data collection, and analytics without touching the core control system. You get immediate benefits like improved operator efficiency and predictive maintenance capabilities at a lower initial investment.

Here's the real analysis: The S7-1500 upgrade is a complete modernization with higher upfront costs but better long-term ROI (3-5 years). The hybrid solution has lower initial costs but creates technical debt - you're still maintaining that aging S7-300 hardware that's becoming harder to support.

My recommendation? If your S7-300 is still reliable and you're budget-constrained, start with the hybrid approach to get immediate benefits. But plan for a phased S7-1500 migration over the next 2-3 years. The hidden costs of legacy systems - downtime, scarce parts, safety risks - will eventually outweigh the savings of keeping that 90s technology running.