Some people just don't realise that Phoenix and MSL are two very different missions, designed for very different goals.

MSL is designed for long range, long duration field geology with a secondary astrobiology mission, and is an expansion of what the MERs have done. Phoenix is designed to test very specific hypotheses about soil physics and chemistry. MSL cannot test them, it does not carry the right payload.

Trajectory considerations limit MSL to +/-45 degrees of the equator. MSL would not be able to be sent to the regions that Phoenix can reach (~70-80 degrees N). MSL would be useless through the polar night and would be a wasted mission for much of the time, even if it could be sent to the polar regions

Phoenix will cost US$284 million, MSL of the order of US1.2 billion, more than four times as expensive. For the specific questions that Phoenix is designed to answer, it is the superior probe. There were hopes to test the Viking hypotheses about the nature of the martian regolith hypotheses with MPL, Beagle 2, and Mars 96, so there are a lot of people waiting on this. Plus those who had hoped to see their instruments fly on the 2001 lander.

Phoenix is an excellen mission that will provide some long overdue information on aspects of the Martian surface, will explore hithertoo unvisited regions of Mars, and so so very cheaply.

Jon