The 5D and the FP have the same adjustment range and are the same size, but the FP has click adjustments. IMHO, the FP is worth the extra money. I have the FP on my 39A. I didn't need to change the front sight at all. I also have a 5D on a Winchester trapper, and if I could do it over again, I'd get the FP. The 5D has served me well, though. Once the 5D sights are set, to shoot it isn't really any different than with the FP. The FP is just more handy for fine adjustments, that's all. As far as being "bulky", well, it is a little bit. Once you get used to shooting with the Williams reciever sight, though, you'll never notice it.
I have the 5-D on my 39-A and like it a lot. Doesn't seem that bulky to me copmared to the Redfield 70 Target that I had on the rifle previously. As Hogship mentioned, fine adjustments can be time consuming. I use the friction of the lock screws to keep movement from getting out of hand when making fine zero adjustments.
Incidentally, this sight was referred to by William's as the 5 Dollar sight in the mid 50's.... changes in pricing must have shortened the nomenclature to simply 5-D.
If you are worried about bulk check out XS Sight's ghost ring for the 39a at www.xssights.com. The set includes a front post sight, two different sized rear sights, and a punch to remove the existing rear sight. They also sell a threaded rear sight that will accept the inserts from the William's sights.
If you go with a receiver sight I second the opinion that the FP is worth the extra money. I found adjusting the 5D to be a real pain.
The XS sight is easy to adjust and in my opinion it looks cleaner on the gun than the receiver sights.
I also have a FP on my 1897 T. I also replaced the front sight on mine with a fire sight. 8) Much improved. Maybe not quite as "rugged" as the barrel mounted sights, but much better to aim with. :wink:
The Williams 5D looks a lot bulker than they actually are. One of my 39A's has one and it's staying right where its at, on the rifle. These work just great and once you have the thing set where you want it and tighten down the screws they stay put. The inserts change easily enough and if you want a bigger sight window just remove the insert altogether and use it as a ghost ring. Changing inserts takes all of about 8 seconds.