Hello. I was in your same shoes last week, so thought I would offer some advice from the research I have done. I will premise this by saying that I am in no means an expert in PDAs. I just received my hx4700 today.
When comparing the products online looking at specs, the hx2750 appears to be the clear winner. It has more memory than any of the others. If security is a big issue, then the fingerprint reader on the hx2750 is really nice. The reviews say that the hx2000 series is bulky, but the hx2750 model is thinner than the hx2410. I didn't find the 2750 to be bulky at all. The screen is nice, though not as sweet as the VGA on the hx4700. The hx2750 does not have the best battery life, but it is ok.
I have not seen the Dell Axim in person. I did not end up choosing the dell because their customer service sucks. I will never buy a dell product again due to past experiences. Someone else who is not biased against dell should talk to you about it.
I was
set on the hx2750 before I went into a local retailer. I just thought I would get a feel for the products before deciding for certain. Once I looked at the hx4700, I could not put it down! The screen is INCREDIBLE! It is so crystal clear. I also found the touchpad to be very cool. Some people have said they have difficulty controlling the movements with the touchpad....but so far I like it. I had no issues using it at the store. I will have to let you know how I feel in about a week

The memory isn;t as large on the hx4700, but it is sufficient for my needs. The battery life on the hx4700 is also rated to be awesome...which was also really important to me as I often forget to charge. I thought I might have an issue with the size of the 4700, but it actually is the perfect size for me.
I would recommend that you try to compare all of the products in person. That really helped me decide. Have you thought at all about the 4700? I hoped this helped.