I see trouble brewing with this question!
for whats its worth my opinion is this....the 4 cylinder engines are lighter in weight leading to a better balanced car all round, they have 50kg less over the front axle but have much less power, however with some modest suspension upgrades and a fogged airbox, and if you want to go the whole hog a re-map and performance exhaust the M44 engine can end up with around 155 BHP and have fantastic handling...you could also change the rear diff ratio as well, but even without any of this I find my little 4 pot really good fun to drive and its nice to have a car where its possible to floor it without losing my licence!...just the fogged box will give you better throttle response and make the car "snappier"..and give maybe a 5 BHP improvement..
if you like old english sports cars and have a more refined view of what a classic car should be then the 1.9 will be enough, its a really nice little car that really makes me smile every time I drive it...I tried a 2.8 as well and really do not regret my choice.
now if you want to turn that smile into a maniacal grin, then the M is the car for you!,I have driven a mates one and have to say the performance is really impressive, bags of torque, great handling (although I think the 1.9 can be made to handle as well if not better) and acceleration that is an instant facelift!..it is a made for purpose sports car and lives up to its well deserved reputation.
however I personally would not swap my 1.9 for one unless I was allowed to sell it to fund buying another 1.9 and a 2.2...but then I come firmly from the "little sports car camp"
my favourites of all time are either a toyota GT2000 or a triumph TR3..just so you have an idea of where I am coming from.
whichever you choose will be bags of fun but the M will have higher running costs no doubt, from insurance to fuel, parts and servicing..both are good mileage for the age so you cant really lose...
are you a thrill seeker? or do you just want to cruise about with a smile on your face? , your answer to that question will give you your best choice!