I've been giving this a lot of thought, as my situation is not dissimilar to yours.
My main concern would be the combination of open plan, plus risers and a back row. For me, I'm not sure if that'll look great visually. If you do want two rows, I would go with the dedicated option, personally. Conversely, if you go open, I'd stick to a single, wider row.
Some things I've had raised to me, which are all great points you might want to consider (not all of these are facts):
1 - Open plan rooms generally will get more use. In the end, a room is only as valuable as the time you spend in it. Particularly if you're not intending to watch solely movies.
2 - With open plan you have a couple of audio challenges. Firstly, noise floor. The whole basement has to be quiet if you care about high quality audio. That means taking isolation / insulation seriously. Secondly, physical space. You'll need higher end, higher quality - louder speakers to achieve the same volume given the larger space. However, when you achieve this, sound in a larger space can be superior.
3 - Not having a back wall can be an advantage and a disadvantage. It's not an acoustic disadvantage in itself, as not having a back wall prevents reflections from front to back. However, it can make speaker placement more problematic.
4 - Design is a bit more problematic when you want to incorporate the cinema into a more general living space. You want to minimise reflections (in order to maximise contrast) and that means dark colours which don't tend to fit well with living spaces. I'm going through a design process on this exact thing right now. Trying to minimise compromises I'm making, but they're there.
5 - Light control. With it all being one big room, you would need to darken the whole space in an open plan design. Again, not impossible - just something to consider. This is assuming you want it to be immersive. If you don't, then many of these challenges go away.
In my personal basement project I am trying to use the 'darkness' a home theater requires as a way to partition off that area of the basement. I'm automating light control across the whole space, and isolating the whole basement to have the same noise floor I would in a dedicated theater. I'm also upgrading my LCR speakers and subs, which have to fill a much larger space. I'm using a single row of seating (potentially with a bar behind later) to make it more accessible and comfortable.
A preview of the design is attached. I'm playing with adding black walls & carpet for the first 1-2m to really enhance that 'floating screen' look. I'm also going to experiment with making the soffits black. When I have a full set of renders, I'll attach them to my own thread and share that with you here.