Buy an ASIC and you will be stick with one algorithm forever, for example if it's scrypt ASIC you will have to mine only scrypt algorithm coins

Buy GPU and you are free from ASIC restrictions, GPUs works on many ASICS available, from Ethash to kawpow, autolykos2, cryptonight and many more

Truth is there are few Ethash coins left and if ETH goes dead today your ASIC won't be profitable because no other Ethash coin is very profitable like kawpow or autolykos2 (ergo)