Eurostar is the concession that operates the Channel Tunnel (Chunnel). Eurorail (or Eurail) is the organization that creates railpasses for most of Europe, basically offering railpasses for select or consecutive days. All traffic through the Channel Tunnel is on a Eurostar ticket. If you are seeing a price difference, I am assuming that the Eurorail ticket is "cheaper", but only because it assumes you are using a Eurorail pass that you already paid for in addition to a supplement (the cost you are seeing) to get a ticket. If you are only going between London and Paris, you do not need a railpass, so just book a Eurostar ticket.
As for your options, on the way over, if you do not have the time to spend a night or two between England and Paris, then why take a leisurely route? If you do, then it would be a good thing.
As for flying, I assume your flight is out of Heathrow for an international flight? If so, given needing to be at Heathrow at 9:00 AM to check in, I would think twice about leaving Paris that morning to catch a flight at noon. Why? Figure 9:00 AM check-in, conservatively 1 hour to get from St. Pancras station to Heathrow (putting you at 8:00, first Eurostar train arrives at 8:12), your 2.5 hour travel time - 1 hour time adjust, meaning a 6:30 AM departure from Paris Est station, which means a 6:00 AM checkin, which gets you leaving your hotel catching the first Metro train that comes by. Not impossible, but cutting it very close. Flying is no better, most cheap flights will go into Stanstead, Luton, or Gatwick, not Heathrow...figure a good 1.5 hours to transfer between airports, roughly an hour flight, meaning you need a 6:30 flight with a 5:00 AM check-in, leaving your hotel at leasdt by 4:00 AM, well before the Metro runs...meaning a very expensive taxi ride, if you can get a flight that early. Consider spending the night in London, taking the train the day before your flight.