The question is ‘what is your end delivery format?’
If it’s the internet, then I’d just go with the native frame rate of the file and change the timeline in the NLE to match.
If you’re working for TV, however, you’re going to need to get that frame rate to 29.97
If the After Effects project has no prerenders or 25fps imported footage, then setting an alternative frame rate in the render settings will have no detrimental effect on render times or final quality. There is no interpolation going on. After Effects is just measuring a different point on the timeline.
However, if there are prerenders included in the project, then it becomes more complicated.
One way to do it is to think about the PAL footage as though it were film footage and then transfer it to NTSC as you would a feature film, using a 3:2 pulldown.
To do this, render the project at it’s native PAL. Bring the footage in to a new project and interpret the frame rate of the footage as 23.976. Now make a 29.97fps timeline and drop the footage onto it. Rendering the timeline using fields will now give the footage a 3:2 pulldown. Use upper field first unless you are rendering to a DV codec and interpret accordingly in your NLE.
This will actually slow the timing of the footage down by 4%, but seeing as this is how feature films are shown on European PAL TV (by speeding them up by 4%) it’s a completely standard technique. You won’t notice it in the picture, but you may notice a pitch shift in the sound. If you do, then you may need to time stretch the sound separately, using an app that can do this without altering the pitch.
Of course 3:2 pull downs suffer naturally from frame jitter, since the cadence of the frames is a little strange… every third frame is on screen for a touch longer. But this is nothing that the US audience won’t be used to, since they’ll see this on everything shot on film and broadcast on TV.
If you really want to avoid 3:2 pulldown techniques, then you could try interpolating frames. AE doesn’t do it very well. You might want to look at Re:Vision plug-ins for this, but even so, none of it competes with the power or speed of a Snell and Willcox machine… and indeed if it’s a pro solution that you require, then this is the direction that you should go.
Hope that helps.