First of all … you should model what is observed NOT what is accumulated . Secondly an ARIMA model can evolve into a time trend model with Intervention Detection with the potential of detecting breakpoints in trend. Stay way clear of simple ols models with trend or trend squared unless theory ( domain knowledge )tells you so .


Closely review a piece I wrote contrasting and comparing ARIMA with Regression a few years back. https://autobox.com/pdfs/regvsbox-old.pdf