They both adjust liquidity, but serve different functions. A splice directly modifies the channel and its size. So I can splice more sats into my channel and it will in turn increase that channel's total capacity. Splicing out will also decrease the channels overall capacity. Swaps leave the channel state alone. With swaps you're simply moving liquidity through swap providers to swap in and out while paying them a modest fee. With splices it's a cooperative agreement with your channel partner to increase or decrease the capacity of the channel by effectively resetting/updating the funding transaction. Downside is you need both channel participants to run a lightning implementation that supports splicing. This can also help clear a looong list of old commitment transactions for a given channel.