Two seniors traveling one way. Thanks.
Train - absolutely. Just over two hours and no airports to mess with. Buy tickets early - they are less expensive that way.
The train is the only way to fly. Quicker and cheaper if you take advantage of advance, discount ticket sales.
No brainer - take the train-er!
The exception is if you are flying into London and then going immediately to Brussels, in which case if you can get a flight to Brussels from the same London airport it will be more convenient than the Eurostar.
For the price it depends. plane might be more expensive or cheaper depending which airline you take and also what London airport you are arriving at. London city is the most convenient of all.
Train is arriving in central London so very easy and convenient, less time to spend in check in etc.
I agree with the comments stating that it really depends when you buy your tickets. We're flying into Brussels for 50 Euros each on Brussels Airlines.