Cycling: SPDs vs Clips - which is better?

I'm sure its a question asked many times... As it happens I can do a small test.

When I first got my current bike I wore toe clips, and did 25 rides using them. Then, changing nothing else on the bike, I switched to Shimano SPDs. In that configuration I did 40 rides.

So the two configurations were the same, apart from the pedals.

Here's a table

	Clips	SPDs	SPDs "better" ?
n	25	40
total distance km	902.76	1450.55
overall av move speed kph	25.84	25.84	0.01
stdev ride move speeds kph	2.38	2.71	0.32
watts / hr mean	1.45	1.46	0.01
average power	175.00	168.53	-6.48
average heart rate	121.92	118.75	-3.17

So how about do they make me faster? Well, taking the total distance ridden and dividing by the moving time, nope, almost no change.

Here's a frequency breakdown of the average moving speed of those rides, normalised to take account of different sample sizes.

Did they cause me to produce more power? Well in terms of watts / hr the answer is, again, no. Maybe this is a function of the kind of riding I did?

What about average power (as given to me by Strava) ? Actually average power went down - shouldn't it go up?

And finally, heart rate (as measured by the Garmin HRM) to produce that power and speed... well I guess good news is that it went down over 3 bpm, though that could be just ascribed to increasing fitness?