Mill circles came really easy for my dd - so I would have to say front hip. Even those weren't too bad though - just don't ask how long it took her to finally do a good kip with straight arms and legs, lol.
DD has been struggling w/ front mill circle since October . She just about has it - it is just not pretty and not yet consistent. She has only tried the front hip circle at one practice - so at this point I would have to say mill circle - I hope the front hip is not as difficult for her
My dd had to do the mill circle in her l3 routine and it toke about 2months but once she learned it she hits every time ,she is now in l4 and has been practicing the front hip for 3mon no go yet but getting close.
Both of my girls learned their stride circle before their front hip. In both skills they need to roll their wrists to the top of the bar. The front hip was harder for them because you have to time it just right and be aggressive to get around the bar.
I took DD (7) from Sept to last Wednesday to get her mill circle. She did back in Jan get one in a warm up before her level 3 meet, but then did not have it til this week. We are going to open gym today so it will be interesting to see if she does it again.
Front hips are still in progress. They have only been working on them since the end of Feb. Her level 4 season will not start until Jan '09, so hopefully she will have it by then.