The main difference between R30 and R40 is the size of lamp, lamp wattage,trim and fixture. Today lamps sizes are available to fit into almost any downlight fixture that will allow flood, spot or narrow spot lighting. Positioning of the fixture will ultimately be deceide by the position of the joists in the ceiling and available attic space.
What do you want the down lights to do? Lighting task area such sinks and counters? If you are using downlights as a task light, ensure the beams overlap to reduce shadowing. General kitchen lighting? Flood lamps with higher wattage at the door, refrigerator and counters. My favorite combination for down lights is 75 watt halogen spots all on a dimmer. I usually follow the counter line with 1 fixture on each side of the sink. Add to this a down light (on a separate switch) at the door of the room or directly in front of the refrigerator (makes a great nightlight and midnight snack light). I then add under counter fluorescent fixtures, mostly for task, and then add a few strips over the cabinets for indirect lighting. This gives a kitchen many diferent lighting schemes for a varitey of moods. Good task for cleaning up and preparation, good spots on dimmers for setting mood just as you light the candles but enough light not to trip over everything getting the food to the table, and indirect lighting for general walking around with out wanting light in your eyes.