First of all what lvl are you?
Since I'm drawing near to the lvl of scarecrow training and being on a fairly populated server what possible alternatives would I have to those places besides MP?
Well, Haunted Mansion is the best exp so you won't be measuring up, but if you're really having difficulty getting any of it with overcrowding than there are a few other places that are still okay, I trained partially at scarecrows/hoodoos/voodoos/jesters but I went to some other places when they were really too crowded. Eos Tower can be okay, keep going lower the more you level though. Then at around mid sixties maybe head to Omega Sector for a bit, then Verne Mines In Edelstein at early 70's until you get to about level 80. Then head to magatia and you could be there til late 90's if you wanted to. That's largely what I did. (though I moved to Galloperas at 91) Malaysia also is okay from like 45-95 with all the different guys there .
You can go to Magatia as early as mid 60s on most chars; iron mutae have a wonderful spawn for AoE skills. Stay there until your 90s and then move to Captains/Kru.
Instead of scarecrows I go to Robos in ludi (aparatus room). I honestly don't notice much of a difference in exp rates there, especially if crows are crowded.
Other than the occasional late night toy room over the summer, I never train at Masteria. The other areas aren't nearly as bad as some people say, as long as you know the right maps.
Spoiler!
Well, I just hit 43, and the lvling at scarecrows is painful, since Iron Arrow has no vertical tolerance what so ever scarecrows actually seem kinda slow.
since you said 43, ludi in the clock tower would be the best bet for you
CD training is wonderful for any horizontal mobbing class. Since Iron Arrow has no real vertical range, the map setup is ideal for Crossbowmen. It's one of the best training spots for late 30s into mid 40s.
Spoiler!