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.
You can always take a chance with every pair but that will be applicable to someone who knows how to trade and I am sure the OP may not be in that shoe. But why stress myself anyway since I eventually still trade ETH against BTC and BTC being the legacy coin, which virtually 100% of other altcoins are traded with, then I do not see why U should choose ETH pair over BTC pair.
As long as OP is buying the alts to hold, what differs? However, if he is a trader and he is trying to trade a specific pair based on the reaction to it and whether it is acting bullish on that pair or bearish, then he can decide to go ahead with any of them, at the end though, it is all about the decision and what he was holding to buy those pairs but it does not really change anything. For instance, an altcoin may be acting bullish on Ethereum but not on bitcoin, so it all depends on skills of smart trade.