Approximate transition energies and oscillator strengths are computed with large-scale multi-reference configuration interaction calculations to differentiate between linear pentadiynylidene HC₅H and its C_s symmetric isomer ethynylcyclopropenylidene HC₃C₂H by certain fingerprints in their electronic spectra. The first dipole-allowed transition of the HC₅H spectrum is obtained at 2.76 eV (³Σ⁻_u ← X ³Σ⁻_g) in reasonable agreement with experimental measurements of 434 nm (2.86 eV) in neon matrix. The oscillator strength of ³Σ⁻_u ← X ³Σ⁻_g of linear HC₅H is computed with f = 0.007, a value similar to what we obtain for linear HC₇H (f = 0.005). In addition a second very strong ³Σ⁻_u ← X³Σ⁻_g transition is predicted by the calculations around 7.3 eV. The yet-to-be-observed electronic spectrum of HC₃C₂H is characterized by four strong transitions calculated at 4.79 eV (2 ¹A″), 6.17 eV (2 ¹A′), 7.27 eV (3 ¹A′) and 7.74 eV (4 ¹A′). The first transition of HC₃C₂H 1 ¹A″ ← X ¹A′ is calculated to be very weak and at much higher energy (3.64 eV) than the first ³Σ⁻_u ← X ³Σ⁻_g transition of the linear isomer.