Same as you, 1/4" x 20. I use Allen head cap screws, and the tenon is only 1/2". Yes, that leaves only 1/8" on each side, but my tenon is only 1/4" anyway, and most of the insert goes into the neck. You're thinking,"That's going to crack, all the time." It does, so when I thread the insert, I use a channelled block that wraps around the tenon, with large holes to thread the insert through (think of a mortised neck block, that has holes where the inserts go through). This prevents it from cracking, but still pushes the tenon out, which for me, makes the joint that much tighter.
I've used the flat heads in the past, but with pickup installations, the battery bag doesn't stick well. The cap head (with a star lock) is what I'm doing currently. I used to cut the velcro backing for the battery back to go around the Allen bolts, but with the Five.0, not really an issue - it fits right inside. You'll like two bolts, trust me.
For the OP and for conversation, the reason I did the M&T is not necessarily for "strength" or whatever, its what I think Rick referred to in another thread - there are just things that make my current builds "complete", and this is one of them.
Unfortunately, when it comes to the neck, I'm "locked" into my jigs, and how the "centerline" application carries into shaping jigs, routing jigs, and even fretboard jigs, and if I want to do something different, I have to do it by hand. However, it took me a long time and a number of builds to stratify what I do all the time, and what I don't. What I do all the time is make necks, in the same dimensions, so being "locked in" to my jigs works for me. In the end, the player won't know the difference one bit whether I use a bolt on, M&T bolt on, screwed, splined, etc.
mzuch - my neck angle jig is almost exactly Robbie O'Brien's, except smaller, and made out of Baltic Birch plywood, which is pricey, but its what I use in my Bass cab, Guitar cab and speaker cab building. Once I used real Baltic Birch, all other plywood is just, plywood. Note: I was going to make it a neck jig, instead of a neck angle jig. I'm glad I put the angle part in, as I'm not a machine, and no matter how well I jig to make the neck block square, its not, so I can cut the M&T to match the body. There are pics on the Big Island `Ukulele Guild website photo gallery, start from pic #22
http://www.bigislandukuleleguild.org/gallery/ugh---exhibition-2013.html