On the matter of cost, DH is a standalone for $10($5 during the steam seasonal sales) while HoI3+all 4 major DLCs costs $30 although I do have an old HoI3 steam key I could part with that would cut $10 off the price. by researching them thoroughly and gather a team of the army. Players can develop tanks, majestic planes, etc.

All major powers have been vividly portrayed. (And it sounds like HoI4 is bringing that back, grrr) Gamers get thrilled when it comes to experiencing the advanced technology system that developers have incorporated in Hearts of Iron IV. I also think the HoI3 tech system is exceedingly superior to DH's 'Tech Teams' BS that screws over minors. The large nations (specifically Sov) can become a micro managing hell, especially in multiplayer but I will take it any day over DH+Kaiserrich. HoI3 puts a greater emphasis on flanking and creating cut off pockets to destroy and the 10000+ provinces are amazing for that. Don't get me wrong, Darkest Hour is a great World War simulator, if it is the first one that interests you. To be honest I like complexity and HoI3 is far better in that regard due to the increased number of provinces.

It depends what you want, I find that the people who hold up Darkest Hour as superior to HoI3 only do so because they like the reduced complexity.