If you are just experimenting, then it probably doesn't matter much.
If you are seriously trying to develop a business application, you need to ask a different question.
What carrier has better support for
testing and distributing my application?
As far as maturity with this, I think that NexTel is way ahead of the pack. This would mean that
you should work with Motorola iDEN phones such as the i88, i90 or i95cl. The big drawback to NexTel/Motorola is that they only support over the air provisioning through their proprietary technology. They do not support downloading jar's via a jad file.
If you want to work with Nokia, join the ATT developer community. I belive they support the standard over-the-air downloads as described at
http://wireless.java.sun.com/midp/articles/ota/ Also, you want to make sure you get a platform that is big enough for what you want to do. Color is also a feature you might want to consider.
There is also the Motorola T720 offered by T-Mobile and others. But I find it slower than the iDEN phones under NexTel.
Hope this helps,
Nancy
[ February 03, 2003: Message edited by: Nancy Lehrer ]