I believe bitcoin fluctuates less than Eth, so if you're just looking for a coin to trade with alts, it should be BTC. But, I guess you could take a chance with Eth too.
However I find that when Eth fluctuates wildly, the alt eth prices fluctuate wildly too. So there's really no benefit in trying to game the system thinking you can get a better eth price just because you're buying/selling in eth.
I agree with your insights rwgarding some Altcoins depends on how Ethereum fluctuates and the Eth depends on Bitcoin as well and that is what had happened and that is how everything works in this cryptocurrency industry. Though all these things depends on our own preferrences and of course our desire in choosing something that makes us comfortable and safe more importantly in terms of investments.