I can only speak for the Giraud trimmer as I have never used the Henderson, but it does an exceedingly good job on both the inside and outside chamfer. The inside and outside chamfer are adjustable when using the Giraud trimmer, so its output can be tailored within reason to match the specific brass being trimmed.
FWIW - the effect of using different bushings and/or mandrels to size a specific Lot# of brass are really negligible as far as having to adjust the blade setting on the Giraud. That's probably because for a single specified cartridge, the amount by which the bushings and/or mandrels that are typically used don't vary by all that much (i.e. a few thousandths at most). The most important point is for all the cases within a single prep to be trimmed consistently. What I have found to be more noticeable in terms of adjusting the blade setting is the difference in neck wall thickness between different brands of brass. For those that do not turn necks, using a different brand of brass with a different neck thickness may require re-setting the blade position slightly to ensure both the inside and outside chamfer are as desired.
As long as the there is some detectable amount of chamfer on the outside (i.e. the corner has been knocked off to the extent you can see it), and the inside chamfer is acceptable, I do not attempt to re-set the blade position on the cutter head. It's more trouble than it's worth. I simply make sure I use the same blade setting for an entire prep, even if the inside/outside chamfer are a tiny bit different than a prep with a different Lot# or brand of brass. For example, I recently switched from Lapua to Alpha SRP brass for my .308 Win F-TR rifles. The inside/outside chamfers on the Alpha brass were just a tick different than Lapua, for which the cutter head had been previously set, but hey were both quite acceptable, so I left the blade where it had been. The other key to using the Giraud is that you need to spin the cases by hand while they are trimming to ensure that a uniform bevel/chamfer is obtained all the way around the case mouth. I generally do at least three turns per case, maybe four if the brass is being chamfered for the first time (virgin brass).
As far as the two different trimmers causing different seating force, I am of the opinion that something else was likely in play there. There is no reason that the relative amount and/or "smoothness" of chamfer at the very edge of the case mouth should cause a noticeable change in seating force, unless the inside chamfer with one trimmer was set sub-optimally (i.e. the inside chamfer was insufficient). Otherwise, the dimensions of the bullet and chamfer are such the boattail and bearing surface of the bullet is likely only touching the very inner bottom edge of the chamfer during the seating process. If a bullet is seated with more than half a caliber of shank in the neck, a thousandth or two difference in the length of case wall gripping the bullet isn't likely to make a huge difference either, especially as long as all the cases within a single brass prep are the same. Finally, even if the chamfer on one of the trimmers actually did result in a slight difference in seating force, that doesn't mean those bullets would undergo a different force during the actual firing process, which is all that really counts.
