First, if you are going to become a professional pilot, it doesn't matter which one you learn in - after 200 hours, you'll have the same level of expertise.
The R22 has a bit of a stranglehold on the training industry due to the low cost of entry. It is not a superior trainer in any way (in fact it is deficient in some key areas), but they have built a bit of a closed-loop system with the Pathfinder insurance deal. Therefore, you are most likely to find a teaching job if you did your training in an R22, but no matter what the "Robovia's Witnesses" preach, you gain no special skills flying the R22. However, especially in the UK, it is what you'll most likely end up flying.
The S300 types are tough and forgiving trainers - no suprise since that's what they were specifically designed for. The key factors for training safety and robustness are high rotor inertia, fully-articulated rotor system, long-stroke dampers on the skid gear, and almost unlimited visibility from the cockpit. In my mind it is the best training ship out there. It is not the best personal helicopter - that's what the R22 was designed to be.
The 280/28 series is a bit of a different animal. Much more like a "big helicopter" in flight. If you can afford it, it will be a less jarring transition from the Enstrom to a 206, or 350 or whatever. The Enstrom is a very safe ship, but the turbocharged engine makes it a somewhat tempermental trainer - students overboosting, overspeeding and not properly cooling down lead to a lot of stuck valves (lucky the 280 autos really well).
So train in whatever you can afford. In the end, your CFI, and the mechanics who maintain the aircraft will have a much bigger effect on your training then what type of helicopter you are flying anyway.