Ha...never intended to be the "clerkship expert." You can look at some of my other posts about competition - I'd say for the most part, fed district cts are just as competitive as circuit courts (although some districts are obviously less desirable - it's a bit harder to find people to go live in Alaska for 2 years, but not MUCH harder).
Therefore, someone coming from a T-15-20 will need the same credentials as I mentioned in my last few clerkship posts (although some have suggested I've been too lenient in gpa/reank, and they may very well be right): journal, top 10% (probably higher, though), publication and/or executive board position on journal, stellar recommendations, etc.
Having a judge who is an alum from your school, or who has in the past hired from your school is a huge benefit (for example, Judge Carnes on the 11th Cir tends to prefer hiring Emory grads).
As for whether firms give bonuses for fed district courts, it would depend on the firm. I don't think I've ever heard of a firm with a clerkship bonus differentiating between district and circuit court clerkships, but maybe they're out there. I know my firm doesn't, and if you do a federal clerkship, you get a bonus.