9-channel 101 - most bang for buck configuration?

I'm currently running a 7.1 system consisting of the traditional 5-channel + front "presence" speakers (Yamaha receiver).

I purchased the L/C/R and the bookshelf speakers I'm using for the heights as a set (Boston Acoustics), so they all match. The side surrounds are bi-poles of a different brand.

Considering going to a 9.1/2 setup, changing AVR and adding speakers. I know different AVR brands have slightly different configurations for 9-channel.

So, I'm kind of wondering, if I went to a 9-channel, whether I would get the best effect from adding another set of bi-poles in the rear, another set of bookshelves in the rear, a set of floorstanders in the rear, or a set of floorstanders as "wides" in the front. (I am pre-wired for any/all of these potential placements)

Any thoughts?

My room is roughly 19 x 22 - with the TV on one of the 22' walls. Have not measured exactly, but I would have about 3.5 - 4ft distance to the wall behind the main listening position.

Thanks for your vote. Can I ask why you would prefer direct monopoles over bipoles?

From a lot of the things I've read, many people seem to suggest bi- or dipoles on the back wall and mono books on the side. I decided to go with bipoles on the side as my room is not a dedicated HT room, it's a family-type room with various different seating positions. My thinking was that the bipoles would "spread out" the sound a little better for everyone in the room who wasn't seated in the main position - which is my La-Z-Boy recliner

I was kind of thinking monos on the back wall for my setup - partly because I think I can still grab a set of bookshelves that match the 5 in the front of my system for fairly cheap, but also because my thinking is that putting the bipoles on the back wall might have the effect of cancelling out with the side bipoles where their soundwaves meet??

Also, at the rear left of my room there is an opening to a stairwell leading up to the main floor, which would only be about 2ft from the bipole on that side - kind of thinking that the sound from that side might go up into the stairwell rather than "staying" in the room - but I'm just guessing; I'm an accountant, not a sound technician