Be careful, the cheapest set will usually be the one with the worst picture.
Ambilight is a novelty, not without merit because some backlighting helps reduce eye strain. BUT it does rely on reflected light and therefore a corner position or dark wall may effect perception. Also it adds complexity and expense when a low wattage light/LEDs will do a similar job.
Spend the extra money on a 37" you'll wish you did when your paying the TV repair man an extortionate rate to fix a light fitting!