A question about sunlight

It's summertime, and my indoor garden (veg) is getting pretty hot. On some days, with all the lights going, the temperatures touch the 90s. On these days, I shut down my 1000 watt and bring my plants outside. I leave my 400s running, and keep my mother's and small trays indoors. With just the 400s running the temperatures peak at around 84, but usually keep around 79.
Recently, the outdoor temperature has been hot, but the sun has been blocked behind clouds. In the interest of keeping the room at manageable temperatures, I'm still shutting down my 1000 watt during the day, but I'm wondering if anyone knows, is outdoor sun with cloud interference (i.e. an overcast day, where occasionally the sun breaks through) still better than a 1000 watt indoors? As I understand, when the sun shines bright and the temperatures are right, it trumps any amount of indoor light, but I'm curious about the comparison on those less than ideal days outdoors.

You bet it is better, much better. Even though it might not look as bright to your eyes as the 1000 watt you are getting much much more usuable light for the plant. I move mine inside to outside every day, I groan sometimes and tell friends I can't go somewhere got to move my plants at lights out or before depending on the weather. I love putting them in the rain during veg though they really like it.
You just have to maintain a good pest control schedule because of exposing them to the ones staying inside.
Added bonus to your labor, more UVb more frosty buds.

I would think sunlight even on a overcast day is better than any MH/HPS light in terms of spectrum. I don't think it would be much better than LED light though. I use this chart on the aquarium hobby a lot when people try and say MH has a much better spectrum than LEDs, just to show how good the spectral output is with LEDs and how bad it is with many MH. Keep in mind this chart is using an aquarium MH bulb that is rated at 14k, so it isn't a fair comparison to MH bulbs designed for growing, but even grow bulbs have a lot of gaps in the spectrum compared to most LEDs.

This chart compares spectral output of Cree cool/neutral/warm whites to a 14k MH bulb.

Interesting, thanks for the info. It's good to know that even on an overcast day, my plants are getting a healthy dose of light.