I would go with a stacked w/d rather than an all-in-one unit. My thoughts are that if one goes bad down the road, you only have to replace 1/2 rather than the whole.

Either way, 28" depth can be difficult to find. What have you found for both options?

No machine can go flush against the wall, they have hoses and need a little space. I had a single machine that both washed and dried. I never used the dryer. It was too small to dry the full load I washed and took so long I could have washed several loads. I ended up hanging most of my clothes.

Ideally, I would recommend stacked front loading washer and dryer. I had those top-load combo units in apartments and they were generally small and performed terribly. The advantage of a combo unit, though, is that it usually uses 1 plug vs. 2 for separate machines (verify this before buying, of course). Depending on what your electrical situation is, that could sway your decision.