Let's look at the tale of the tape: Microsoft Lumia 535 vs. Microsoft Lumia 540 Dual SIM vs. Microsoft Lumia 550 - GSMArena.com
I'd choose the 550. Unless: If camera is a major factor or you want the option of reverting to 8.1 then go with 540. 535 really shouldn't have anything to recommend it unless the price is that much better.
- Age - this is easy, 550 is newest and was made for W10M.
- LTE - only 550 has this.
- With a slightly smaller screen (@ 4.7") the 550 is smaller and lighter. It does gain some thickness though. Might help with grip?
- 535 loses on screen resolution. The other two are about equal in screen.
- The 550 gains a Snapdragon 210 SoC which might appear slightly slower on paper, but is more efficient and capable (LTE - Video Playback) https://www.qualcomm.com/products/snapdragon/processors/comparison
- 540 has better cameras (front and rear)
- 550 has slightly better GPS and Bluetooth protocols
- 535 is the loser for battery. On paper the 540 has the largest battery. The 550 is only 100 mAh behind, but the more efficient SoC should make it come out on top.