I generally don't look at the thread category ... just the thread title

In a TT there's really no substantive difference where you register. You can always see everyone's results, so if you are interested in comparing your time to the times of other people you will be able to. If racing masters means getting a prize, why not go for it.

In a road race it does require a bit more thought. The masters fields generally throw together everyone - so that's 35+ cat's 1-4 (the most we ever break them down up here is A's -35+1-3 and B's- 35+ 4's) so you will likely be racing with some very fast women and the field sizes are generally smaller, so it usually will be a harder (though possibly safer - as the racers have some experience) race in the masters than in the 4's. Up here at least, we see few women's masters fields offered in any case - generally just state championship races and some TT's. There's just not enough time in the day and racers available to justify all those fields.