I'm playing SH3 + GWX3 or SH3 + WAC5.01 as well as SH5 + TWoS, I like all very much and I can give you some meaningful differences:
- SH5 + TWoS has the top grafics of the series and all sub interiors present that work flawlessly because already designed for the stock game;
- GWX3 has a lot of historical scenarios added to the game, but so does SH5 with its campaign and the TWoS mod; it would be very subjective saying that one is better than the other, so I'm not going to try.
- SH5 + TWoS is top notch in realistic play (you can chose 'Real Navigation' or not) with some nice additions like Automation Features that will allow you to automatically run an evasion pattern, navigating out of ports, or an automated mix of submerged and surfaced operations. There is nothing like that in other SH series;
- The campaign in SH5+TWoS is something to get used to, but you have total freedom to play like you want while certainly keeping the level of all historical scenarios present in GWX3. In SH5 you can accept missions and receive special missions during your patrol which is not true in SH3 where you are only ordered to patrol a quadrant, but in both cases what counts at the end is your tonnage. In SH5 there is also 'KSD Commander', an equivalent of SH3 Commander adding further to setup your campaign the way you want, the savegames are highly editable too (spawn date, spawn at port etc.)
- SH3 has the better crew management features, in this regard SH5 is more limited
- SH3 can be played 'faster' in that you can use keyboard shortcuts to quickly change the stations. In SH5 there is a 'Teleport' feature but all in all you need to walk more through the compartments, but that may be what you want, more sub compartments mean more immersion. Also the deck gun/AA gun operations are more complex and not as easy as they should in SH5.
- A result from the last 2 points above, from my personal experience SH5 is 'harder' to play than any SH3 + megamod, but it's also more eye-candy and rewarding. But this is not a judgement, only my experience.
I hope this helps you, I wish you fun with both simulations, they are certainly all worth to install and play!
Cheers,