I reckon the Revell kit is good for detail, but has a couple of inaccuracies, although they don't spoil the overall effect. The Airfix kit, if anything like the standard Lanc they released in the 1980's is worthwhile, and accurate, although lacks some of the fine detail of the Revell kit.
That's a very good, brief description. Agree wholeheartedly.
At the end of the day, you have a choice. Either try to correct the old tool Revell (a bit of work, especially in the fueslage), or get any of the newer kits. Each has it's faults, but almost all offer a fairly decent representation of the Lanc.
Available 1:72 Lancs that I know of:
Airfix - Mk.I/III ('60's), Mk.I/III ('80's), Dambuster (reworked '80's Mk.III with Dambuster components)
Hasegawa - Mk.I/III, ASV Mk.VII, Dambuster (all approx.'05)
Matchbox - B.I/III ('70's/'80's)
Revell - Mk.I ('70's?), Mk.I/III ('05), Dambuster ('93 - old tool with Dambuster components), Dambuster ('05)
Myself, I've made the old tool Revell, and Matchbox kits, and have the newer Airfix and Revell kits (as yet unmade). IMO, the best in general shape and proportions is the Airfix; as Terry said, the Revells shine in their details.