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.