Firmware is dependent upon hardware. The Marlin and Repetier products were both written for 8-bit Arduino boards, although I hear that Repetier, at least, is now available for some 32-bit ARM boards. (I don't know if that's just beta or if it's been fully released.) Smoothieware works on 32-bit Smoothieboards and its clones, and RepRapFirmware is written for 32-bit ARM boards (AFAIK, primarily the Duet products and its clones). Thus, if you already have a controller board, you'll be limited to firmware that can run on it, unless you want to buy a new board. If you don't already have a controller board, you can pick anything you like, but the firmware and controller board are at least partially linked, and the quality and features of the hardware can be as important as the firmware.
I've never had a delta printer with an 8-bit control board, but my understanding is that delta calculations push those boards to their limits, which can degrade print quality and/or limit printing speed. (That said, I've heard rumors that Repetier firmware is slightly more computationally efficient in its delta calculations than is Marlin, but I have no way to confirm that, so take it with a grain of salt.) Arduino-based boards also usually lack features like built-in networking (Wi-Fi or Ethernet), which is something I wanted. Thus, when I built my delta printer, I investigated 32-bit products. I ultimately settled on a Duet WiFi, and I have no regrets about that choice; but it was a close call in my mind between that and a Smoothieboard. As an added bonus, I eventually swapped out my original effector with a Delta Smart Effector, which is a product of the same company, so configuring it was fairly straightforward. I don't know offhand what it would take to configure one of these effectors (which includes a built-in bed sensor for auto-calibration and bed leveling) with something other than RepRapFirmware. Duet and Smoothieboard are both expensive options (around $170), but to get the equivalent network control features in an 8-bit board, I'd have needed to add a Raspberry Pi to an Arduino-based solution, which would have narrowed the price gap and added complexity. Of course, your needs may differ. If you're on a budget and/or already have an 8-bit controller board, buying a 32-bit board may be out of the question.
