Of those listed, I have used the BD Alaris, Sigma Spectrum, and the Plum, though not in the prehospital environment. I have used the Medsystem III / MiniMed III (back when it was branded as such) but since those are on the way out, I won't recommend them. I very much like the Sigma Spectrum for its simplicity. The one big downside to it is that if you're considering doing multiple drips, you will need multiple pumps as they are all single channel. The Plum is also very good too but, like the Medsystem III series, there is a bit of a technique to ensuring that the cartridge does not entrain any bubbles or the system WILL stop working. One of the things I do like about this system is that the cartridge allows concurrent infusion of a "secondary" line. For those instances where I'd infuse potassium or some antibiotic or whatever along with NS, I could just run the NS on the primary circuit and the rider on the secondary channel. This minimizes the amount of tubing between the patient and tubing if you're running primary/secondary drips. Like the Sigma Spectrum, they are, however, a single line from the pump to the patient, so if you need more lines for whatever reason (like incompatibilities or requirement for dedicated lines for a med), you must have additional pumps to make that work.
The Sigma Spectrum won't draw from a syringe but for field use, that's fine because you normally shouldn't need to use a syringe infusion pump system in that environment and if you do end up needing that, there are dedicated, stand-alone systems available. One of the advantages of this system is that it is quite compact.
At work I use the Alaris system. Easy to use, prime and so on but there are some downsides to this. One of them is that each "brain" can only handle a maximum of 4 channels. The system is rather large and cumbersome and needs to be mounted on something to prevent kinking of the tubing. If you need to run something concurrently with another line, you need to use a dedicated channel and primary line setup to do it. That's a lot more tubing to get tangled in stuff. It's not the best for the ED but it is reasonably decent and the rest of our hospital uses them on the floors. Because of this, it is easy to standardize on tubing and so on, and this makes it easy to transport patients that have ongoing infusions to their destination floors/units and just leave "our" pump and take one of "theirs." If tubing compatibility wasn't an issue, I'd also go with the Plum for ED work. They're simple, reliable, small form factor, and rugged.
While I would prefer the Plum system, for most prehospital systems, I'd suggest going with the Sigma Spectrum and have 2-3 units available. Unless you're doing multiple drip interfacility runs, that's all you're likely to need. If you're doing multiple drip interfacility runs, I would suggest getting the same pump system as your primary clients as this minimizes problems when swapping drips from a hospital's system to yours. Also if you need to "borrow" pumps from the hospital, you will already be familiar with that system. The second system I'd recommend for prehospital (not IFT) systems is the Plum, though again, you'd need to have a couple on hand.