Why either or? I consider it both.
Most of mine is stashed away as an investment asset, and rest is used as a currency when I feel like using crypto. Notably it would be stupid to do your grocery shopping with Bitcoin right now, use it where it gives you an advantage of some sort over other methods of online payment.
The fact that the value change (positevely) over the time is also the sign that this is a strong medium and not only a currency.
Almost all currencies change their value a lot, so this is not a point that disproves Bitcoin's status as a currency.
To me, an asset that is scarce and increases in value over time like bitcoin. Spending it is a waste, instead we should utilize its full potential to create greater benefits.
If you spend some, just buy more. That is an easy solution to what you wrote.