Kelpies and Falkirk Wheel from either Glasgow or Edinburgh

I think it's six of one, half dozen of the other. It's a little closer to Edinburgh, so it might save you 10 minutes, but I would just opt for whichever is most convenient with your schedule.

Have you considered adding it to a trip to Stirling Castle? You can get to the town of Stirling directly by train from either Glasgow or Edinburgh. It's a little closer to Glasgow and would save you about 10 minutes from there. So you could go to Stirling first, head up to the castle (taxi, bus or walking), spend some time touring the castle and then the town, for which I would recommend at least three to four hours. Then make your way down to The Kelpies and Falkirk Wheel from Stirling, which would take about 50 to 60 minutes. Then head back to whichever city you came from.

The X37 bus from Glasgow claims to serve The Falkirk Wheel on it's timetable.

In truth there is a degree of optimism in that claim as the stop is 15 minutes walk away, because the stop is at the bottom of the access road- it runs hourly.

You can actually walk there along the canal literally from the end of the platform at Falkirk High station, on the fast rail route between Glasgow and Edinburgh.

As you can see on the schematic below the frequent buses 7 and 8 run to the wheel itself from the town centre (for which Falkirk Grahamston station, on the Stirling line) is the better station.

The buses 2 and 3 have a stop 20 minutes walk from the Kelpies, although the F14/F14A get you rather closer. You can also walk there from Grahamston mainly along the canal.

There is also a bus, the X38, to Falkirk Town Centre from Edinburgh, but it isn't the fastest way to get there by any means.

For an area map and timetables see- https://www.mcgillsscotlandeast.co.uk/services/falkirk

There are also two very different bus routes- the F16 and the 38 between Falkirk and Stirling. And yes I have done all those routes including the X38 from Edinburgh, then the F16 to Stirling, back on the 38.