Was surprised when I saw this, since I own both a GK NEO 412 and both the Bag Ends you're considering. This may be my first post where i actually know what I'm talking about!
First off, these are two fairly radically different choices. The GK, loud, grindy, plenty of low end, a bit too much sometimes, a killer rock band cab. It is large and heavy, close to 95 lbs, but I still find it fairly easy to load in my truck unassisted. I turn 60 years young this year, btw. I really love this cab, despite the size and weight. One of the better "rock" cabs out there, IMHO.
The Bag Ends are an entirely different beast. Very clean and true I think. What tone you put in, that's what comes out! They don't produce tons of deep low end, but they really do shine in a full band mix. The S15d is an amazingly loud cab for it's size, but it's not going to blow the walls out in a club. The S15B-D does help with adding more low end "oomph", but it is large and fairly heavy. Once again, I love both these cabs. I also have a S12D that I couple with the s15d on occasion.
The Bag Ends will give you more options when it comes to gigging different rooms. If it's a small club, the S15d by itself will probably cover the job just fine on it's own. Pair it with the B-D and you're ready for some pretty major volume situations.
It's a tough choice, that's why I own both. I prefer the tone of the BE's most of the time, but in louder rock type situations, I still break out the GK if I'm wanting some really aggressive tone for the night.
BTW, if you shop hard, you can find the BE's sometimes for stupidly cheap prices. I have a total of about $350 in all three of my BE's thanks to getting lucky and finding them priced WAY below value on GC's used website. The S15B-D was only $49.99 with $20 shipping from Cali to Oklahoma.
The other two were priced @ $125 and $149. Absolute steals! I would have paid MUCH more for them and still have been happy. Best of luck on making your decision, both your options are good ones.