Put the old DIY under water to see if it leaks. You can have leaks from any seal and the tubing itself can leak.
The Hagen sstem hangs on the tank, so no siphoning when you open it up. You can see the bubbles so you know when it slows down. System seems to not leak, but I'll admit I haven't actually tested that underwater.
Downside; cost of replacements, size of unit, possible algae on diffuser, leaves in diffuser making bubbles stop.