There Are Pandas, and Then There Are Pandas.
And this isn't either of them! The Pandas we're talking about here, are watches, not bears. And what got me thinking about them (again) was a link posted this morning by @cm.rook who pointed a few of us to the very attractive (and not terribly priced) Yema "Rallygraph" Panda which, in it's most traditional arrangement, looks like the one on the left, but can also be had in the version on the right: The model on the left is a true Panda, while the model on the right is called a reverse Panda. The reason for that distinction is clear--Panda bears, only come in the first arrangement. Now at this point, everyone should be thinking about the most well-know Panda, The Rolex Panda, which is actually a Daytona, and among Rolex Daytonas, the most famous of which is the Paul Newman Daytona, which was famous first, because it was Paul's, and second because it sold at auction for $17.8 million (US Dollars). The story of that auction is well-known so I'll only...
Nov 8, 2019
I'll add how I figured out which lens to buy - I rented the lenses I was thinking about. If you live in a large city, there is most likely a professional camera store where you can rent for the weekend (they almost always do a deal, pick up on Friday after 3PM, return on Monday by 10AM and you only pay for one day). Even if it isn't exactly the lens you are thinking of, it will give you an idea. There are also national camera rental companies that are a bit more expensive, but they work with FedEX.
For years I drooled over an f2.8 300mm lens. I wanted it in the worst way. I was almost ready to buy one. But then I discovered renting. I rented one for the weekend for a football game. I learned a $5,000 lesson - I didn't really want one of those. It is way too big, way too heavy, and too much work to use. Bottom line, it was work to get it out. Even though the pictures were stunning, some of the best I've ever taken.
I then rented a f2.8 70-200mm for the next weekend. That was the perfect compromise. Bought a new one a few weeks later. Saved $3,500.
And don't forget the L f4 lenses - they are also super awesome. And the advice to buy used is outstanding. It can be really hard to find used f2.8 Canon lenses - they disappear quickly. And you'll never win the lottery for refurbed f2.8s - those get snapped up the minute Canon posts them.
If you're drooling over a 5D or a 7D, go rent one. Those bodies are way way heavier than your current camera. You will either hate them or love them. But you will know how they work for your given situation.
Honestly, I did this backwards from the advice you got here. I upgraded the body before the lenses. But that was a photography choice, the things I photograph told me to do that.
Only you can answer what you want to do with your photography. Is the body your limiting factor right now, or is it your lenses? Think about how many times you take low light pictures. My guess, you'll discover not nearly as often as you think, and only you see the dead pixels (especially if you aren't printing the pictures).
Good luck.