Easy: flowering light
Replacing a 400W HPS with either a 600W HPS or 400W of LED should - all things being equal - result in about a 50% increase in yield (as long as you have the space).
So there you go - I've just paid for my new light with enough left over to buy a new veg light, too.
In almost every case, upgrading your flowering light - if it is sub-optimal to begin with - will result in an increase in yield. That buys a lot of electricity. It will also likely improve your smoking experience and save you time in trimming, by growing denser, higher quality flowers with less popcorn to trim around.
What does upgrading your veg light do? Seriously. You are going to flower at the same stage of growth. Getting a better veg light may get you to that stage quicker and save a couple of $ in power. It may reduce stretch, giving you a bit more head height. But if you run a perpetual cycle indoors, you have at least six weeks to veg in-between cycles (eight weeks flower less two weeks to clone or raise seeds), which is plenty of time to get to the required stage of growth with modest veg lighting.
A better veg light is not going to improve your flowering yields. Unless, as some here have mentioned, you plan to grow outside. Or use the same light to veg and flower.
But if you're using the sun to flower, or one light to veg and flower, then we're not even having the same conversation, are we? You only have one light, so there is no choice as to which to upgrade first. I don't even know why these questions were asked, hence why I didn't answer them above. It's obvious.
Without sounding like a broken record, I just don't follow the OP's logic. A better flowering light will always give you better yields, which is money in the bank to upgrade your veg light. You wouldn't put the cart before the horse, would you?
And again, I am not arguing there are not efficiencies to be gained in upgrading your veg light. I'm simply arguing there is a logical (fiscal) order in which to do it.
As for CFL's, they have their place. I'm using one to supplement UV and vertical light in a staggered grow. It was cheap - $28 or something - and LED doesn't have much UV to speak of. The buds in these photos were harvested this week and were so big when they finished that they broke quite a few branches:
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