From a safety/cost perspective, I'd suggest you mount elements in your keggle if at all possible. My experience with heatsticks (having made 4 of them) is that they have a lifespan. I used different methods, mostly the pour in method, but based on my experience, they will likely not last forever. The pour in method with a j-bend heat stick was the first to go. It's pretty hard to thin JB Weld enough to be effective pouring into a J-Bend.
That being said, if I was making a heat stick today I'd go with an all copper one. There was a design on her somewhere. You can use silicone tape/wrap on the handles so they don't get too hot.
Heatsticks are very flexible. But what's your setup like? Honestly, it's no more expensive and only slightly more work to mount your elements. Plus, there are additional safety concerns with heatsticks as you are submerging your wiring. My experience was that they worked very well and were flexible, but once I had one fail, I started mounting them in my keggles. Luckily, when mine failed the gfci shut it off right away, but it was disconcerting.