If you feel that underwired bras don't currently do anything I would suggest getting a good fitting and seeing what it should feel like. The difference is really pretty noticeable. If you still don't feel they are better then stick to what you like, but I'd try a good one first.
Maybe try John Lewis for a fitting, they are normally pretty good and have a good range of sizes.
OK...if you were fitted at M&S I will take a bet. It's wrong. M&S use a stupid and outdated method of measurement which results in very badly fitting bras. I'm a 30HH ish. M&S would have me in a 36F. That's how wrong the get it.
According to their chart. a 34A measures 29.5 under and 33 over. In reality someone with those measurements is more like a 30C or D, or even a 28DD. Yes, honestly...
34 A, and just before giving birth when I was fitted for a nursing bra - at M & S. Don't have a tape measure to hand for the measurements... left that to them. Maybe as a smaller chested person its not so important... feel happier without the wires but just don't want to let everything slide south....
An underwired bra is for lift and support - and on larger breasts is kind of essential for comfort and avoiding back pain too.
However...you say you can't see a difference between wired and underwired which leads me to the inevitable questions...
What size are you wearing? When were you last fitted? Where were you last fitted? What are your measurements - measure tightly under your bust (no bra) and then more loosely over the widest part wearing the best fitting bra you have.
I could be wrong but I suspect if you can't see the difference the underwired bra might not be up to much.
I feel a little foolish asking this, but I do want to know.... What is the purpose of an underwired bra? I have a small chest and don't see a difference cosmetically between those and the soft, wireless ones I wore during pregnancy, which are more comfortable. My friend says my boobs will sag more if I don't go back to underwired. Is she right?