First of all you will have to be sure what you want to do a bsc or a btech if you want to do bsc then go for bhu while if you want to go for btech then nit and iiit allahabad are both good colleges and here i would advise you to go with nit allahabad as first of all you will get a nit tag plus coding atmosphere here is top notch and placements are great and improving each year hence decide wisely.