The brother 1034D is a great workhorse machine.. I had one for years and never had a problem with it other than the cutting knives getting dull (totally normal, I replaced them a few times when I had the machine), then I had the chance to try one and I fell in love with the Babylock Imagine and had to have one.. it's so easy to use, there's no thinking or fiddling with tension involved, you just set it up for the stitch you want and it comes out perfect every time. I've had mine for about 7 years now and I'd absolutely buy another.
To be honest the air threading.. I could take it or leave it. Once you know how to properly thread a serger it's really not hard to do and it doesn't take much time at all. I think the people who find it so difficult just can't follow directions (the order in which the loopers are threaded matters a lot, and not all machines are the same). What really sold me on the Imagine was the auto-tension.. it's like magic
I also got a really good deal on a Pfaff 4874 several years ago and I've had a love/hate relationship with that machine... it serges really nicely but I mostly use it for coverstitching and there are many times I wish I had something simpler like the Janome Coverpro that only did coverstitching, so that I wouldn't be constantly re-threading the machine every time I changed my mind about what I wanted to do with it.
What it really comes down to is price and ease of use. I recommend the brother all the time without hesitation, but I admit I am a bit spoiled with my Imagine
The Pfaff... I probably wouldn't buy another.