Fillet and sirloin get treated as a status contest. They are not. Fillet is the lean, tender steak that hates being overcooked. Sirloin is the steak with a fat edge and more flavour for most British palates. At a sit-down table, pick the one the people in the chairs will actually enjoy. Fillet steaks and sirloin steaks are both honest if you stop using them as a personality.
Fillet when tenderness is the point
Fillet Steak 2x 6oz is two proper fillets. Hot pan, short time, probe 50–52°C for medium-rare, rest. There is no fat to rescue you. If someone at the table likes well done, fillet is a poor gift; it will eat like an expensive mistake. A whole fillet is a different cook — a roast — and lives in whole beef fillet. Do not buy a whole fillet because the 2x pack felt too small unless you can roast and slice it.
Sirloin Steak 2x 8oz is the steak most guests recognise as a steak. Render the fat, rest it, and it looks like a restaurant plate without the fillet price. For a table of four, a 6x 6oz or a family 8x 6oz sirloin pack is more useful than four mixed ones-and-twos. Sirloin freezes cleanly. Fillet does too, but you have paid more to freeze it, so only do that if fillet is genuinely the household favourite.
A mixed table
If two want fillet and two want sirloin, buy both 2x packs and cook them in the same pan with staggered times — fillet off first. That is kinder than forcing everyone onto one cut. What you should not do is buy fillet for people who wanted fat and flavour, then drown it in a sauce to make it interesting. Sauce is allowed. It is not an apology for the wrong steak.
Price without snobbery
Sirloin feeds a habit. Fillet feeds an occasion, or a household that truly prefers lean tenderness. There is no prize for the dearer pack. There is a prize for empty plates and a pan you did not burn. Buy the cut you will cook properly. That is the only table that matters.
If you are still unsure, buy sirloin. It is the steak that teaches the pan and feeds a mixed table without sulking. Come back for fillet when someone at the table has asked for it by name. Named want is the only good reason to pay fillet money, and it is a perfectly good reason when it is real.