I think the bitcoin price is more important. It means it is taking more market share from bitcoin.
Depends if you're trading/gambling to try and use alts to build up your bitcoin stash, or investing long term in the overall crypto-currency concept.
Pricing in dollars is basically a measure of the total value that *all* crypto, combined, is absorbing from the old-world, dying, fiat-currency global system.
Both measurements are important...
Ultimately, if you're cashing out - or at least want to know what your stash is actually worth, then USD is what you value it in.
If BTC tanks to $500, then it won't matter of the XMR/BTC exchange rate is twice as good, you will end up with less fiat. Until we can all pay our bills in BTC, or (better still) Monero directly - the USD rate does matter and it is still how we work out the 'real world' purchasing power of our XMR. There is no getting away from it - you can pay for some things in BTC now - but is still based upon the rate it converts into fiat.
So it IS wise to think in terms of a fiat value. We all put fiat IN at some point to get started, knowing how much we have increased this value if we cashed out is a measure worth knowing.