I don't follow cryptocurrency news much, and don't really understand the consequences of this. I do have some BTC in Coinbase, though I don't really spend it. Would you recommend I withdraw it ahead of the fork?
A very small futures market is predicting about 20% of the current BTC value to go to BCC. If that's correct, you'll miss out on about 20% of the value of your holdings if you don't withdraw prior to August 1st. This number is very uncertain though, I think it could end up being anywhere between 5% and 50%.
In my opinion, market cap is too superficial to capture what happens in the market. A better measure would be market depth: how many coins can you sell into the market before the price goes to zero? This figure very well might split into two, since otherwise it would require action on part of the market makers, i.e. entering the BCC/USD or BCC/BTC market.
If you have it on your own, you can technically use it on both chains independently. The history breaks August 1st. From there are on it is basically two currencies.
Though in general you should keep your bitcoin close to home. If Coinbase gets hacked or just ceases operations your money could just be gone if the fallout is lost in litigation hell.
In 20 years maybe it won't be so uncommon to have 20TB lying around, but I know I don't have a lot of things on my computer that take up more than 1TB and only ever grow. I certainly won't have two of them...
I'm not being disingenuous, I had coins in a cold offline wallet, and it was a serious endeavor to get them out to where I could spend or cash them. I had to spend days downloading a block chain and the only thing that kept me from freaking out was that I knew this would work, having done it plenty of times when the chain was shorter. I couldn't find any way to retrieve coins from a wallet.dat without downloading the block chain first. I had thousands of dollars tied up for days while I waited.
I can't imagine what it would be like when the block chain was 8x its current size, or if I had more than one cold wallet and I needed to empty all of them in any kind of a hurry (like, in time to capitalize on a bubble before it pops!)
You basically get free money if you withdraw, because you can split the coins so that they can be spent on both chains. But you need to be able to do this without messing anything up, which has gotten more difficult since 2013 or whenever.
Wallets have new features to protect privacy, but the trade off is that they are more dangerous to work with. A few years ago the best practice was to make a cold wallet and then delete the private key from your computer, but doing that with a modern wallet can result in losing all your coins.