There have been a couple recent threads on this, try a search to find them. The pros are more power than the original engine, more durable, certain parts are easier to come by and cheaper, and it will leak less oil. The downside is that the engine feels different, looks different, sounds a little different and it could detract from the value of the car, depending on who the next buyer is.
A good compromise is an early 3-main 1800. You get the power of the 5-main 1800 but retain the feel of the original MGA engine. You also get to keep the mechanical tachometer instead of having to use a later electric unit. I forget which engines put which starter where, but I seem to recall that the 3-main 1800 will allow the starter to remain where it was, where the 5-main will require a modified back plate. Someone may correct me on this issue.