Given that the Platinum #3776 and Sailor 1911 product series both cover so many different models – with different materials, trim and finishings for the pen bodies, as well as sometimes the design of the cap – I think perhaps you should be more specific about which models you want to compare; for example:
all being full-sized models in plain black resin with gold trim, and with the broadest selection of gold nibs in various nib widths and types†. Otherwise, there is really no contest and I'd say the Platinum #3776 wins hands down; I have eight (not counting my fiancée's) pens in that series and would happily buy more, just for the beauty of the different pen bodies.
Firstly, I don't like the Pilot FKKN-12SR-B-* (of which I have two, both with nibs I don't enjoy using, as it turns out). It has the longest but thinnest bodies (capped) of the three, and I don't like the knob at the end of the clip. I don't have a Pilot 14K gold #5 Fine nib on either of those, or my Pilot Custom Heritage 91 for that matter, to compare against other 'Japanese Fine' nibs, but after being a big fan of Pilot in general and having spent thousands on other Pilot fountain pens, I find the Custom 74 completely underwhelming. As a product line, it also has the most boring colour options.
Then, the Sailor 1911 Large in black with gold trim. Its shape and girth are more to my liking than the Pilot Custom 74, but I still find the pen body boring as hell. A (1.)plain, opaque black finish on a (2.)lightweight resin body in (3.)cigar shape with (4.)gold trim all at once is a formula to turn me off, even though the individual aspects can work in other combinations. I put up with my black-GT 1911L because it is married to a (yellow) gold Naginata Concord nib, and I don't want to buy another 1911L in a different colour with gold trim just to swap the nib or section across. (I did buy a black-GT Sailor Profit REALO, which is still sitting sealed inside a plastic sleeve untouched since it left the factory, originally with the intent of swapping the nib or section across.) It is the only model of the three to offer a Zoom nib as a standard option.
Lastly, the Platinum #3776 Century 'Black in Black'. It has almost exactly the same form factor and shape as the Sailor 1911L, has a Slip and Seal mechanism in the cap as a major selling point (in its marketing, at least, regardless of whether the Sailor and Pilot pens actually dry out more quickly without such a feature), and the resin is ever-so-slightly translucent, even though it is the cheapest of the three in terms of street price. It is also the only model of the three to offer an Ultra Extra Fine nib as a standard option.
So, to me, Pilot Custom 74 is the clear 'loser', and the Platinum #3776 Century has just that much of an edge over the Sailor 1911 Large – before taking into account nib differences, and notably that the 1911L comes with a 21K gold nib as standard, whereas the others come with 14K gold nibs. Don't get me wrong, I enjoy writing with the Platinum #3776 14K (yellow) gold F nibs, almost as much as much as I enjoy the ruthenium-plated 21K gold F nib on my Sailor Professional Gear Imperial Black, but the number of carats may make a difference to some people, especially if they also want to take price differences into consideration.
Disclaimer: I don't actually have any of the three with equivalent nibs to do a fair head-to-head comparison.
† Let's ignore for now the additional nib types only available on the Pilot Custom Heritage 912 or better.