Is it easier to get last minute bus tickets or train tickets for day trips out of Barcelona?
I would not say easier more like there will be space available and that the bus depends on where specifically you are going on a day trips.
You can monitor the Renfe website to see whether trains are selling out ("Tren Completo"). That only applies to the fast trains (AVE, ALVIA, Talgo EuroMed)). If you're taking a regional train, there are no assigned seats and sell-outs are not possible. Use Barcelona (Todas) as your origin.
I've seen inter-city buses sell out occasionally.
Where are you thinking about going?