Hi,
1) The two roller adjustable will give you more control over the crush resulting in a finer overall crush. In practical terms that means you gain a few percent of efficiency in the mash. However, that's not a big deal as you can compensate for lower efficiency by adding just a bit more grain.
The corona mill has crude wing nuts to adjust the grinding gap, but with a little tweaking and experimentation is acceptable.
2) One other minor difference is that a two roller mill will leave more of the husks intact which can help when forming the grain bed for sparging. Again the effect is relatively minor for most homebrewers but can help overall efficiency and also help avoid stuck mashes.
Overall the corona style mill is an acceptable and cheap alternative - I used one for several years with good results. I eventually moved up to the barleycrusher which I really love (mine holds 7 lbs at a shot, it is quicker, gives you more control, is beautifully constructed, can be set up to run off an electric motor).
Just like most equipment - there is a trade-off between dollars spent, convenience, time and quality of product. The decision depends on how much you want to spend, how often you brew, etc...
Cheers!
Brad