Both research and personal experience are pretty clear on this point: teach to the gifted side, and provide the supports they need on the challenge side so they can manage the workload.
Of course, these kids are never that simple
so a couple of extra thoughts. For mine, more complex content is critical to keeping them engaged and motivated enough to work through the pain of their disabilities. This is especially true for my kid with attention and writing issues, as these create barriers to doing just about any kind of work. Nothing sets off his AHDH worse than work that is repetitive or uninteresting but makes high demands on his disabilities: it will take him a million years to complete it.
Another important consideration is, is your TAG program actually a gifted program - i.e. more complex content for kids who think differently? Or is it a high-achievers program, with lots more of the same kind of work, and the expectation that the kids will do it in larger volume, faster and with less help? The former can be magic for 2E his; the latter is less likely to be a good fit.
So my best suggestion is to think about what makes your child happy, excited and engaged, and what kinds of supports she needs, then try to get as much info as you can to figure out which place is more likely to be able to get closer to meeting both those needs.
Be prepared to make a lot of trade offs, and to change schools again as her needs evolve and different trade offs make more sense. I find it helps when I am really honest with myself, too, about what some of those look like: taking lower marks as a consequence of gifted content, for instance, or taking longer to graduate. The more flexible I can be, and the less expectation I have that our path will be straightforward, the easier it is to help my kids work through the curves. I think the is especially true for me, as my own non 2E school experience was so different from my kids.
