2.5 diesel or 300tdi

i have a 1986 defender 90 2.5 PETROL! i have had it for about 4 months and i reckon it is only doing roughly 8-10 mpg. so i am looking at a conversion. i have found two engines locally...
*2.5 turbo diesel (roughly late 1980s) out of a defender with 100k mileage
or *300tdi (1996) out of a disco with 180k mileage

the diesel is roughly half the price. i know the diesel would more or less fit straight it whereas the 300tdi would require alot more work. however the 300tdi would do more to the gallon and still have more power and speed.

what would be your thoughts?
which one should i go for?
how bigger job is it too fit a 300tdi?
what mileage would i get from each?

thanks

op2:

Fit the 300 at any cost, don't put a td in, they are rubbish.

The td will get you low to mid 20's, the 300 should get you 30-32 mpg. It also actually works, which the td won't for long. It also actually has some power meaning you don't need to change down into first to go up a 1:100 slope.

Just do the 300 - or alternatively, even better, wait for a good 200 which fits straight in with virtually no modification.

I have a td.....and i am going to convert. If you drive it often/daily its not for you. Mpg isnt great. If you are going to just do field work itll do. I am going with 300tdi. Lil bit more refined ( slightly) and more power than 200 i believe.

Same power, just more electrickery to help keep the emissions in check. I prefer the 200 but they are getting a bit long in the tooth now.

Figured that. And i wont be wading through mud to be honest. Mines road, woodland and occasional greenlane type stuff.