Trainers can be really good especially if they know you well. Also ask if they have any bits that might work spare so you can try it out first. My old trainer had a treasure chest full!
Id prefer my trainer, and by watching other horses at shows that look similar to ride like yours..
Am still proud of the fact my trainer told me I had done very well to find a bit/mart combo that my horse went well in..*sigh* little things...
Your trainer - particularly if they know you and your horse well. They may be able to lend you a bit to try first otherwise it gets rather expensive !!