Recording in a bitrate-reduced format like MP3 can be advantageous especially for field recordings. You can get much longer recording times for a given amount of storage compared to linear PCM (WAV files). And they are faster to download via the Internet. However, if circumstances permit, consider recording in linear PCM anyway.
Think archive when you record, even just for capturing the moment. You never know when material may be needed again.
The quality of PCM is much higher than that of a bit-reduced format. PCM is generally easier to edit, and can, when needed, be readily converted to bit-reduced formats—those that are popular now, and those that will inevitably come along in the future.
Even though PCM requires more storage space, computer storage costs are ridiculously reasonable, with increasingly more bytes per buck, and this will likely only get better each year.
But it’s with the portable recorder that you may run into limitations. Even if they have internal storage, look for units that can accept memory cards, like SD or micro SD. Check if there is a maximum amount of storage the device will work with.
And invest in the maximum amount of storage to maximize your recording time. Memory cards are frequently discounted.
Set up your setups in advance, before going out to record, so that when you hit the record button you automatically record in PCM at the sample rate you need.
Routinely transfer your files to a computer or hard drive to make room for more recordings. Or just invest in more memory cards and keep the originals (well labeled and well filed, of course).
If circumstances are such that you have to record in MP3, use the highest bitrate possible, to get the best quality out of that format.