I agree with the above, I have the same rod and fish the 7/8 line with it.
Could I cast heavier flies and tips with the 8/9? Yes - but for me that's not what I bought the rod for.
Foremost I would think about what flies and tips you would be planning to throw with it. For me I use the rod in Low Water with small tubes, doubles and hitches where I much prefer the subtle landing of the 7/8 line. If you are planning on using copper tubes and/or heavyish T-Tips then you are probably better off with the 8/9.
The best option of course would be to buy both
Steven.