The first picture below shows a Perrin figure between two GHQ figures. The Perrin figure comes from the ACW021 - "Iron Brigade, Marching" pack. The GHQ figures are from the ACW43 - "Marching in Sack Coat & Forage Cap (USA)" pack. The rest of the pictures show different angles. I apologize for the poor lighting, but I think you can still get a pretty good idea of the detail on the figures.
As you can see, the Perrin and GHQ figures are approximately the same height, but the Perrin figure is much thicker and bulkier, especially when it comes to the rifle. I think the GHQ figure looks much better, but the Perrin miniature does have advantages. Weapons getting bent is a huge problem with the GHQ figures, since they are more to scale and the rifle barrels are therefore extremely thin. Every part of the Perrin figure, even the bayonet, is very sturdy and won't get bent from handling it. I think the GHQ figures look good enough to be worth the extra trouble of having to bend the weapons back into shape often, but others may not agree. I just make sure to put a couple extra layers of varnish on the places that I think will get bent often to give them extra protection.
Because of the obvious differences in the bulk of the figures, they probably wouldn't look too good next to each other on the same base. They might look close enough once they're both painted, but I still think they'd look different enough to not blend in well in the same unit. I do think they are close enough in size that they'd look okay in different units in the same army. At least, I will find out eventually if this is the case, once I paint and base a unit of those Perrin Iron Brigade figures.
I hope this comparison was helpful.