They both have abysmal reliability, especially right-time performance, which can have a significant impact on passengers making connections at intermediate stations. Given that Virgin Trains has very modern rolling stock and runs on a network that has received substantial investment in recent years, Virgin Trains should be more reliable than East Coast. It isn't. Therefore, I would say that East Coast is better for reliability.
Virgin Trains has some excellent value Advance fares and has kept them at the same price for many years. East Coast Advance fares don't tend to go as low as Virgin's and have been steadily increasing in price over recent years. However, East Coast has reasonable value peak time fares when booked sufficiently in advance, compared to Virgin who have laughable peak time fares no matter how far one books in advance. On balance, I would say that East Coast is better for Advance fares.
Virgin Trains has good value off-peak walk-up fares. East Coast walk-up fares are expensive in comparison, especially north of York (compared with north of Preston). Virgin clearly has the best value off-peak fares, although this is a historical anomaly of the WCML and is not of Virgin's own making. If Virgin had their own way I am sure they would hike the price of off-peak fares, as they have alluded to in the past. On balance, I would say that Virgin is better for off-peak walk-up fares in reality, but philosophically it is a tie (for an example of a different philosophy I would tentatively point to Govia).
On-board service is variable with both companies - sometimes excellent, sometimes terrible. However, East Coast don't make any outlandish claims about their on-board service. Virgin does. The reality doesn't live up to those claims. Therefore, I would say that East Coast is better for on-board service.
Overall, I would say that East Coast is better than Virgin Trains, but Virgin Trains does have some good points which East Coast lacks (some of which I haven't mentioned in detail - e.g. TOC only fares - because of their lesser importance).