And the can is open.......... :-#
If you can - drive a good 300/200Tdi and a good Td5...
Performance - In my experience they drive differently. I find a TD5 a bit 'snatchy' but feels more powerful. But as has been stated - a TD5 can be easily re-mapped to suit your application. Perhaps go on which you prefer the feel of?
IMHO, the running costs are similar but... Given the 'scare' stories of TD5 electrotricarry you may actually get an early TD5 for less money than a late 300TDi so that may save a little.
Reliability - unless you take it to a completely unsupported location (eg Sahara), without spares, then the 300Tdi may be slightly better BUT the 300Tdi does have its own foibles (Cambelt pulley and p-gasket). Overall reliabilty probably depends on how its been looked after and how well you look after it!
So - no help really!! :P My advice - buy the best overall 90 (If thats what you need/want) - Both engines are great.
Ps - I will qualify this by saying that I own a soon to be 200Tdi Series.... But a good friend owns a 300Tdi 90 (On my advice since she wants to go the Sahara). At work I drive TD5s and Wolf 300TDi's