Why is that ? i mean you can use 3G on 4G mifi device and maybe when they would expand their coverage more, his area might also come under 4G coverage and he would already have 4G device and can use 4G than.
Also the packages are same so that means he wouldn't have the regret of using 3G and paying for 4G, if at the moment he is not under 4G coverage area.
I would recommend 4G mifi device.