Mothball,
Perhaps I could offer a Crab aircrew perspective...
Both Obs and WSO offer excellent career prospects and each have pros and cons.
RN Obs training is probably more demanding than that for RAF WSO with a steeper learning curve throughout flying trg. Once qualified, as an Observer you'll be guaranteed a rotary slot with the ASAaC and small-ship (Lynx) roles seemingly being viewed as the most challenging and varied in comparison to ASW. That said, Merlin is seeing a rapid expansion of its role away from pure ASW/ASuW to that a more rounded Intelligence, Surveillance, Target Acquisition and Recce (ISTAR) capability with new sensors (in a similar evolution to that which the RAF Nimrod MR2 fleet is currently undergoing). I suspect that within 10 years, the FAA Obs will also be operating UAVs from afloat.
On the down side, the variety of postings is less in the RN and sea tours will soon lose their appeal when you pick up wife and kids a few years down the line. Some naval aviators I know also express frustration at the career primacy of 'ship-drivers' and warfare types in comparison to themselves. However, I'll let the FAA posters here comment further from first hand knowledge.
In contrast, RAF WSO trg has more varied posting opportunities although as RGP suggests, there are now very few WSO rotary slots indeed. On qualification therefore, you will be either posted to fast jet (FJ) or multi-engine (ME). The FJ route will from about 2009 consist only Tornado GR4 although there is the possibility that some Typhoon twin stickers will be used operationally in a mission command/EW role with a WSO in the boot. Most RAF WSOs will therefore go ME and this will predominantly mean ISTAR. Primary first tour types will be the Nimrod MR2/MRA4 (conducting ISTAR, ASW/ASuW, and with a very useful stand-off strike capability), Sentinel R1 (Airborne Stand-Off Radar (ASTOR), a mini-JSTARS ground surveillance, recce and C2 asset) and AWACS (airborne C2 and battle management). With the advent of A330K, C-130J, A400M and C-17, there will be few if any WSO jobs in the AT or AAR role. Later tours could be on the Nimrod R1 SIGINT asset or as a UAV pilot/sensor operator (some WSOs are already filling such jobs). Quality/stability of life is arguably better and there is a more accepted aircrew career structure.
One final consideration, if you apply to the RAF and don't quite make the grade for WSO at OASC, you may well be offered NCO WSOp. This has some excellent jobs on similar types to the WSO but in addition you may also go rotary on SAR or SH helos. For those with the requisite aptitude and determination, there is a good record of WSOps being commissioned as WSOs later in their career. The RN too have NCO aircrewmen on ASW helos and Commando 'junglies', but I'm not sure how these are recruited or selected. Again, perhaps one of the FAA chaps here could advise.
Hope that this is useful. Whichever service you choose, I wish you the best of luck...it's a great life!!
Regards,
MM