Fill quality will differ as they do not get routed similarly. IB LITE orders get sent to HFTs who pay for the flow. IB PRO orders use our internal Smart Routing which route to pretty much every US exchange and a number of dark pools taking into account, rebates (which are passed on to clients choosing tiered commission), costs, price improvement, etc. Detailed stats on the quality of the Pro fills are published with our monthly metrics.
Beatles - if there was one exchange, Price time priority would prevail. However, with all the internalization rules, multiple exchanges, dark pools, questions/games revolving around the SIP, what qualifies as price improvement, what qualifies as best ex, etc, it is not as simple as "fills are based on first in line rule in the exchange".