Ajman is a good medium and longterm investment.
Prices in Dubai will continue to shoot upwards, and Dubai will be one of the most expensive(it already has started to be) city to live.
Ajman will provide the low-middle class workers (especially expatriates) with housing.
Sharjah remains a conservative emirate where they do not allow booze,night clubs,ect.. so it will not develop as quick as Ajman for social reasons.
Humaid City is ok but if i were you considering you are choosing along the emirates rd, i would choose emirates city, or at least the emirates extension(laketowers)
As for electricity Malaysians are going to build a 2 billion dollar plant to be completed by 2012
Most of the projects in Ajman have R holdings in on it in one shape or form.
The emirate has invested already so much in its market and will not allow the likes of Ras Alkaima steal the show.
Moreover it is possible that the Dubai metro will be at one stage extended, to Ajman, Can you imagine what your flat will be worth in 10yrs time!!!!