I would prefer selling some of my altcoins right now than buying it with bitcoin. Bitcoin halving is very near and it is not really ideal to spent some of it since there is a potential bull run that may occur as halving happens. You should just use your fiat if you are going to but altcoins but make sure first that the value is low so that you can make profit from it as the value rises. Make sure that everything you do with your bitcoin right now is a preparation for the halving so that you won't regret anything as it comes.