Nobody can say what is best so the below is generic only:
If you have legal expense insurance (LEI) and your legal fees do not exceed their maximum level of cover (usually in multiples of £25k, £50k or £100k) and you win you generally keep 100% of the compensation. If you go via NWNF you usually pay a success fee up to a maximum of 25% of their legal fees.
So for example (and full legal costs are not always payable by the oppo so there is always a risk you may bear a proportion of legal fees):
Example Outcome:
Your case wins and you are awarded £100k compensation for your injury and losses. Your lawyers have a bill of £50k.
If LEI your lawyers will seek their legal fees of £50k from the other side and you keep all of the compensation. So you get £100k less your policy excess (usually fifty quid or so)
If NWNF your lawyers seek their legal fees of £50k from the other side and then charge you a success fee of £12.5k (25% of £50k) so you walk away with £87.5k
Insurance usually looks better on paper but you need to check the small print and they will only represent you if you have 51% prospects or above. Of course most lawyers will only take the risk of representing you if you have strong merits too.
I would generally explore LEI as the first option and have NWNF as back up… but in my experience most LEI is a bit shit so speak to both and choose who you think will get you the best outcome after deductions.