If you are considering an ipad or tablet app consider the security. The kit is not specifically designed as a POS and therefore security should be a concern. Far better to choose something that is built for purpose such as the Clover Mobile from First Data. This nifty piece of kit resembles a tablet but is purpose built for handling payment/ordering/stock control etc and has a nice ergonomic handle for making it easy to hold - particularly in a busy environment such as a restaurant.
The fact that it is backed by a major acquirer means that the transaction process is fast, secure and reliable. Plus, you get your money directly into your bank account no fuss. Yes, there are processing charges attached to it so it depends on the volume of transactions you are going to do but for the reliability, security and smooth settlement aspect it is a business decision to be considered.
Simon