Both are EXCELLENT places to start your career, and both hire many ex consultants and produce many experienced consultants. In either case your grad scheme should take you around 2-3 different areas in your first 3 years. Try to mix this up - get some enginnering experience, but more importantly try and get some feel for commerical aspects aswell - project economics, asset valuations teams etc. This blend of technical and commerical will set you up nicely - not only for a faster progression within Shell/BP, but also externally with the consultancies, who will ALL rate experience with the supermajors as a real asset. MBA wise more likely to get funding from BP and Shell but only after about 3 years, and there is increased preference for part time MBA's. In consultancies you will get funding for full time MBAs after 2-3 years, only if its MBB.
In terms of which is better to join. BP is much more commercially active, more deals going on, more aggressively growing, and very interesting strategic partnerships and alliances - eg TNK BP in Russia. However the downside is its going through absolute turmoil internally with a poor handover from Browne to Hayward, and endless fallout from the Thunderhorse debacle. People are all sharp and more commercial/ professional feel to the business
Shell - perhaps slightly overbloated company with 3 people to do every 1 persons job. Everybody has an opinion as to how things shoud work, so very intellectual atmosphere, with the downside that very little actually gets done. Little deal activity in recent years due to stated organic growth strategy. You can get alot of responsibiltiy very quickly, but change is very slow.
Exxon is by far the leader in all measures on any KPI.
Location is the other issue. Shell - HQ in Netherlands - increasingly fewer jobs in London, BP HQ in London. However as an Engineer you are likely to be sent to Nigeria/ Russia/ Oman/ Kazhakstan no matter which company you join so be prepared for this.
Last thing i would say is watch out for the age/ gender profile. Average age in these companies is 40+ and dominantly male, compared to consultings 28-30 - so make sure you network strongly to find your age peer group and balanced mix.
Energy is a fascinating place to be right now - so congrats and best of luck.