If you're intending to buy alts right away then your intent is to increase satoshi value of those alts when bitcoin corrects a bit. It would depend on the speed that you can get from USD > alts through BTC particularly with the current transaction times. I would consider ETH or even LTC (it's also HIGH right now though), because they'll aid you in making a move quickly. Your strategy would be USD>LTC or ETH> Exchange>BTC>alts, but the .25% difference in fees will likely save you a day at current BTC transaction times/regular fees. If you really wanted to buy in BTC, you could buy BTC, send it to your own wallet, then do a CPFP receiver side transaction with a double fee to get it on exchange.