We have been to F&G many times, in all 3 of those months. The answer to your first question depends on personal preferences. Would you rather go when the weather is cooler or warmer? Do you like plantings that are brand new or ones that have matured a little? Do you mind crowds (assuming you want to visit other parks besides Epcot) or do you prefer a little less busy?
Here is what we have learned ~
March: Temperatures range from jacket weather (highs in the mid 60s, cooler in the evenings) to low to mid 70s. Since all the flowers have recently been planted, they tend to be on the smaller side. Still pretty, but no "spreading" to fill in some of the gaps. March is busy crowd-wise, due to spring breaks (each week in March will be a college or school spring break from somewhere!) Good chance for rain.
April: PERFECT for seeing all the flowers and plants in their peak glory. Temperatures are warmer (high 60s to high 70s) though one year it was cold (daytime jackets) and one year very hot (but low humidity, so that helped.) Also good chance for rain. April is also a busy spring break month and since Easter falls on April 5, 2015 you will want to avoid the week prior and the week after at all costs, unless you love CROWDS (and did I mention CROWDS?)
May: Weather has always been mild and sunny, warm but low humidity. Less rain. Far fewer crowds because spring break season is over; however, there are several local school groups on field trips (they are chaperoned and tend to be well-behaved.) The downside is that all the flowers/plantings begin to look tired and overgrown. Still lovely, but not optimal.
As for your second question, Disney always runs specials of some sort but because March/April is spring break season the discounts are not as steep and the rack rates for rooms are higher. So those months will always be more expensive than May.
